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Supriya Mohan Sengupta D.Se., F.N.A. Ex-Professor & Head, Deptt. of Geology & Geophysics, Indian Institute ofTeehnology, Kharagpur

~ CBS

CBS PUBLISHERS & DISTRIBUTORS NEW DELHI • BANGALORE (India)

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ISBN: 81-239-1491-1

Second Edition: 2007 Reprint: 2008

Copyright © Publisher All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without written permission from the author and publisher. Publishing Director : Vinod K. Jain Published by : Satish Kumar Jain for CBS Publishers & Distributors, 4596/I-A, 11 Darya Ganj, New Delhi - 110 002 (India) E-mail: [email protected] . Website: www.cbspd.com Branch Office : 2975, 17th Cross, K.R. Road, Bansankari 2nd Stage, Bangalore - 560 070 Fax:'080-26771680 • E-mail :[email protected] Printed ot : India Binding House, Naida (U .P.)

PREFACE TO SECOND EDITION

It is more than ten years since the first edition of Introduction to Sedimentology appeared in print. It is gratifying to learn that students found the book useful. In the second edition a new chapter on Structures of Chemical and Biological Origin has been added. The discussion on carbonate sediments on shelf has been expanded and most of the other chapters have been revised and updated. Prof. Asru Kumar Chaudhuri (Geological Studies Unit, Indian Statistical Institute, Kolkata) provided useful suggestions for the newly added chapter on. Structures of Chemical and Biological Origin. Dr. Harendra Nath Bhattacharyay and Dr. Prabir Dasgupta (Department of Geology, Presidency College, Kolkata), Dr. Sarbani Patranabis Deb (Geological Studies Unit, Indian Statistical Institute, Kolkata) provided photographs for my use. Dr. Siddhartha Sankar Das (Department of Earth and Planetary Sciences, University of Allahabad), and Dr. Rajat Mazumder (Department of Geology, Asutosh College, Kolkata) provided useful suggestions. Dr. Saumitra Misra (lIT, Kharagpur) assisted in copy editing and proof reading. Mr. Chinmoy Mukherjee drew the revised and newly added text figures. Some well-known publishers permitted me to reproduce illustrations from their publications. Their names appear in appropriate places in the text. Department of Ocean Engineering & Naval Architecture, lIT, Kharagpur provided facilities for this work. Indian National Science Academy, New Delhi, through the. INSA Honorary Scientist Scheme, defrayed the cost of preparation of the manuscript. Mis CBS Publishers & Distributors, New Delhi took active interest in this publication. I am gra.teful to all the individuals and institutions mentioned above.

P-45IB, Hijli Cooperative Development Society Kharapgur (W.B.) 721 306

Supriya Mohan Sengupta

PREFACE TO FIRST EDITION

Sedimentology covers only a small part of the geology curriculum in most universities. Students of geology naturally do not find the time necessary to master comprehensive texts in sedimentology. While gathering piecemeal information from specialized texts they also often fail to appreciate the connections between the various aspects of the discipline. The purpose of this book is to present a concise account of all the major branches of sedimentology (except chemical and biogenic structures) and to highlight the connecting links between them. The presentation is based on my conviction that in spite of the sophistications introduced in the current sedimentological literature, much of the subject can still be learnt by mastering a few basic principles. Once these have been grasped, it should not be difficult, following the references cited, to obtain specialized knowledge on any selected topic. . This book is expected to cater to the needs of students at various levels. The first four chapters are written in a style suitable for the undergraduate students. The first few sections of Chapters 7 (Tectonics and Sedimentation) and 8 (Stratigraphy and Sedimentation) should also be preferably taught at this level. The rest of the book is meant for those at the graduate level. A book of this type could not have been prepared without borrowing heavily from published sources. I am grateful to the authors and publishers who have freely permitted me to reproduce information from their publications. Their names appear in appropriate places in the text. Dr. Indranil Banerjee of the Institute of Sedimentary and Petroleum Geology, Calgary, Canada, critically read an earlier version of the manuscript, provided valuable suggestions, and also made available to me a large volume of current sedimentological literature. I am indebted to him for all his help. J have also profited through discussions with Professor K. N aha of the Indian Instihlte of Technology (IIT), Kharagpur who reviewed some of the chapters. Others who provided information during preparation of the manuscript and read earlier versions of the topics mentioned against their names include: Dr. Sibdas Bandyopadhyay (Indian Statistical Institute, Calcutta; Statistical Analysis), Dr. Abhijit Basu (Indiana University, Bloomington, USA; Provenance), Prof. Ajit K. Bhattacharyya (Jadavpur University, Calcutta; Depositional Environments), Prof. Amitabha Chakrabarti (IIT, Kharagpur; Sedimentary Structures, Tidal Flats), Prof. R. L. Folk (University of Texas, Austin, USA; Carbonate Sediments), Prof. G.M. Friedman

viii

Preface to First Edition

(North-eastern SCIence Foundation, Inc, USA, SedImentary Textures) Dr SIbdas Ghosh (GeologIcal Survey of IndIa, Calcutta), Prof Mandakmi Majumdar (lIT, Kharagpur, Hydrauhcs), Prof. G V. MIddleton (McMaster UniverSIty, HamIlton, Canada, History of Sedimentology), Prof. Asoke Mookherjee (nT, Kharagpur), Prof. H Okada (Kyushu Umverstty, Japan, SII1C1clasttc Sedlmentary Rocks), Prof H. G. Readmg (UnIversity of Oxford, Oxford, UK.), Prof K. K Roy (liT, Kharagpur; Wirehne Logging) Prof S K. Sen (UT, Kharagpur), Dr R N. SIngh and Dr. R. Snnlvasan (NatIonal GeophYSIcal Research InstItute, Hyderabad; TectOnICs and SedImentatIon) Prof. A. Sundborg (Uppsala Umversity, Uppsala, Sweden; HydraulIcs and Sediment TransportatIOn) Profs. A ChakrabartI, C. N Rao and D. P. Sen of lIT, Kharagpur, and Dr D K. Saha of lSI, Calcutta allowed me to use thm sectIons and photographs from thea unpublished collectIOns. Dr. B Mishra helped m photomIcrography Drs Abha Chatterjee and B P. Sandilya provided translatIons for the French and German terms respectIvely I am grateful to all those mentioned above. The contents of thIS book are based on the courses gIven by me at vanous levels at lIT, Kharagpur. I recall WIth appreCIatIOn the comments and suggestions received from my students from time to tIme Some of my M. Tech Students, partIcularly, Asesh Kumar Majl aSSIsted In copy edItmg, Indexmg and computational work at dIfferent stages My WIfe Ila (Sujata) took care of my mundane problems thereby leavmg me free to work on the manuscnpt A large part of the cost of preparatIOn of the manuscnpt was defrayed by the CUrrIculum Development Cell of lIT, Kharagpur under the Quality Improvement Programme (QIP) It IS a pleasure to acknowledge the help and co-operation received from Prof G L. Datta and the QIP staff at varIOUS stages of preparation of the manuscnpt Mr Subrata Gin typed the final verSIon of the manuscript. Mr. Tapan Sarkar drew most of the text figures Mr. S Ghosh proVIded photocopies of the halftone IllustratlOns. But for; the persuaSlOn and Interest of the staff of MI s Oxford & IBH PublIshmg Co Pvt Ltd, thIS work would not have been undertaken. I thank all of them

Department of Geology & GeophYSICS lIT, Kharagpur

Supriya Sengupta

CONTENTS

Preface to Second Edition ., ..................................................................................... v Preface to First Edition .... ..................................................................................... vii

1. INTRODUCTION ....................................................................... 1 Scope of Sedimentology ................................................................................ 1 History of Development ................................................................................ 1 The Early History ..................................................................................... 1 Developments Since Late 19th Century ...................................................... 3 Recent Trends in Sedimentology ............................................................... 5 Literature on Sedimentology ......................................................................... 5

2. PROCESSES OF SEDIMENTATION .............................................. 8 Surface Processes and Rock Weathering ........................ , ............................... 8 Mineral Stability .................................................................................... ..... 10 Source of Sediments .................................................................................... 13 Quartz and Feldspar as Provenance Indicators ......................................... 14 Heavy Minerals as Provenance Indicator ................................................. 16 3. SEDIMENTARY PETROLOGy .................................................... 17 Rocks of Mechanical Origin ........................................................................ 18 Rudaceous Sedimentary Rocks (Conglomerates and Breccias) ................. 18 Arenaceous Sedimentary Rocks .............................................................. 19 Classification of Sandstones ................................................................ 21 Description of Sandstones ................................................................... 23 Chemical Composition of Sandstones ................................................. 28 Lutaceous (Argillaceous) Sedimentary Rocks .......................................... 30 Rocks of Chemical and Biochemical Origin ................................................. 33 Carbonates ............................................................................................. 33 Chemical Sediments Other than Carbonates ............................................ 46 ix

x

Contents

Volcaniclastic Rocks ................................................ '" ................................ Diagenesis ................................................................................................... Diagenesis of Siliceous Sediments ........................................................... Diagenesis of Carbonates ....................................................................... Appendix: Techniques of Sedimentary Petrography ....................................

52 53 54 56 59

4. SEDIMENTARY TEXTURE ......................................................... 61 Grain-Size ................................................................................................... 61 Concept of Particle Size .......................................................................... 61 Grain-Size Measurement ......................................................................... 62 Analysis and Interpretation of Grain-Size Data ....................................... 67 Roundness and Shape ................................................................................. 71 Roundness .............................................................................................. 72 Shape ..................................................................................................... 76 Grain-to-Grain Relationship ........................................................................ 79 Packing .......................................................................................... '"'''''' 79 Porosity .................................................................................................. 80 Permeability ............................................................................................ 81 Appendix: Techniques of Granulometric Analysis ....................................... 82 5. HYDRAULICS, SEDIMENT TRANSPORTATION AND STRUCTURES OF MECHANICAL ORIGIN .................................. 88 Hydraulics of Open Channel Flows ........................................ '''' ................. 88 Physical States of Flow ........................................................................... 88 Structures of Turbulent Flow ................................................................... 90 Sediment Transportation ............................................................................. 91 Sediment Transportation by Water and Wind .......................................... 95 Bed Configuration During Sediment Transportation .................................... 99 Structures Related to Current Ripples .................................................... 106 Structures Related to Wave Ripples ....................................................... 113 Structures Related to Flat Beds .............................................................. 117 'Deformed Beddings .. , ............................................................................... 124 6. STRUCTURES OF CHEMICAL AND BIOLOGICAL ORIGIN ........ Structures of Chemical Origin ................................................................... Concretions and Nodules ...................................................................... Products of Pressure Solution: Stylolites ................................................ Occurrence and Implications of Structures of Chemical Origin .............. Biogenic Sedimentary Structures ............................................................... Stromatolites ........................................................................................ Trace Fossils .........................................................................................

128 128 128 130 131 133 133 140

Contents

7. SEDIMENTARY ENVIRONMENTS AND FACIES ........................ Introduction: Facies Modelling .................................................................. Marine Environments ............................................................................... Continental Shelf .................................................................................. Continental Slope ................................................................................. Continental Rise and Deep-sea Fans ..................................................... Abyssal Plain Deposits .......................................................................... Marine Sediments in Stratigraphic Record ............................................ Non-Marin Environments .......................................................................... Glacial Environment ............................................................................. Eolian Environment .............................................................................. Lacustrine Environment ........................................................................ Fluvial Environment ............................................................................. Mixed Environments ................................................................................. Barrier Island and Beach Facies ............................................................. Stratigraphic Sequences in Mixed Environments ............................... Tidal Flat Facies ................................................................................... Siliciclastic Tidal Flat ........................................................................ Carbonate Tidal Flat ......................................................................... Stratigraphic Sequences in Carbonate Tidal Flats .............................. Deltaic Environment .............................................................................

xi

147 147 147 147 ISS 155 159 160 164 164 168 174 178 190 190 194

194 195 197

198 200

8. TECTONICS AND SEDIJ'vIENTATION ........................................ 207 Sedimentary Basins ................................................................................... 207 Geosynclinal Concept ............................................................................... 207 Plate Tectonics Concept ............................................................................ 213 Plate Movements and Basin Formation ..................................................... 215 Basins in Extensional Setting ................................................................ 215 Basins in Compressional Setting ............................................................ 218 Basins in Strike-slip and Transform Fault Zones .................................... 219 Tectonic Control of Sandstone Composition .............................................. 21 9 Basin Classification and Description .......................................................... 22 3 Downwarp Basins ................................................................................. 223 Rift Basins ............................................................................................ 227 Interior Basins ...................................................................................... 231 Foreland Basins .................................................................................... 234 Subduction Basins ................................................................................ 236 Pull-apart Basins ................................................................................... 236 Delta Type Basins ................................................................................. 239 Composite Basins with Multiple History ............................................... 240

xii

Contents

9. STRATIGRAPHY AND SEDIMENTATlON ................................. 245 Classical Concepts in Stratigraphy ............................................................. 245 Estimation of Geologic Time ................................................................ 245 Dual Hierarchy in Stratigraphy ............................................................. 246 Relationship Between Sedimentary Units .................................................. 248 Vertical and Lateral Relationships ......................................................... 248 Cyclicity ............................................................................................. ,. 256 Correlation ........................................................................................... 259 Sequence Stratigraphy ............................................................................... 267 History ................................................................................................. 267 Basic Concepts ...................................................................................... 269 Terminology ......................................................................................... 271 Depositional Sequences ........................................................................ 271 Application of Sequence Stratigraphy .................................................... 277 10. BASIN ANALYSIS: A SYNTHESIS .............................................. 279 Introduction .............................................................................................. 279 Basin and its Lithic Fill ............................................................................. 281 Mapping, Petrography & Sedimentary Structures .................................. 281 Palaeo slope and Palaeocurrent .............................................................. 282 Depositional Environment ............................................................... " ... 291 Palaeohydraluic Interpretations in Fluvial Channels .............................. 294 Diagenesis and Maturation ................................................................... 297 Sediment Chemistry .............................................................................. 298 Basin Evolution and Tectonics .............................................................. 298 Appendix: Techniques of Palaeocurrent Aoalysis ...................................... 301 References .................................................................................................................. 307 Subject Index ............................ ........................................................................... 329

1 INTRODUCTION

SCOPE OF SEDIMENTOLOGY Sedimentology deals with the processes and products of sedimentation. Sediments are produced either by disintegration and alteration of pre-existing rocks or by precipitation from solution. The particles ejected out of volcanoes and the dust particles of cosmic origin also add to the sediment mass of the earth in a limited way. Sediments are transported by running water, wind or moving ice to various depositional environments. The process of sediment transportation is often accompanied by the production of rhythmic bed forms or more complex structures which, when well preserved, provide clues to the palaeoenvironment and palaeocurrent. Sediments produced out of mechanical or chemical processes are consolidated into sedimentary rocks by the pressure of overburden, recrystallization and cementation. Textures of sedimentary rocks bear the imprint of the nature of the changes ( diagenesis) undergone during consolidation of sediments into sedimentary rocks. The whole process of rock decay, sediment transportation, deposition, precipitation and diagenesis takes place at or near the surface of the earth at normal pressure-temperature lionditions (Fig. 1.1). This distinguishes the sedimentary processes from the igneous and metamorphic processes where a higher order of temperature and pressure is involved. Sedimentary rocks cover wide areas of the earth's surface although they account for only about 5% of the crustal volume. Thus they form only a thin veneer on the outermost part of the earth's surface. The average thickness of the sediment cover on the continental crust is about 2 km but in the ocean basin only about 1 km. Sediments laid down in layers within depressions (basins) in the earth's crust are preserved to constitute a stratigraphic record. The ultimate aim of a sedimentological study is to unravel the chain of events responsible for production of particular stratigraphic sequences.

HISTORY OF DEVELOPMENT The Early History The pioneers responsible for laying the foundation of sedimentology were not geologists by profession. The basic principles of stratigraphy and sedimentation developed out of studies by naturalists belonging to different disciplines. In the mid-17th century, a Danish physician-cum-clergyman, Nicolaus Steno (1638 - 1687), noticed that layers 1

Introduction to Sedimentology

2

of sediments are always laid down in water in a sequence in which the oldest one lies at the bottom and the youngest one on top. This observation led to the formulation of a very fundamental rule known as the Law of Superposition. Steno's second law, called the Law of Original Horizontality, states that the primary bedding of the layers of sediments laid down in water always parallels the surface of the earth. His third law, the Law of Original Continuity, states that all water-laid strata, must continue laterally. A corollary to this law, which came to be recognized by the eighteenth century, states that a truncation of an original sedimentary layer implies removal of the original sediments, either by erosion or due to faulting.

Igneous/ Metamorphic Source

1st Cycle sediments Diagenes' ~---: IS & Uplift Normal PoT

Metamorphic rock ----?1A Greater than normal P-T

Fig. 1.1: Modern version of the fundamental rock cycle conceived by James Hutton nearly 200 years ago (based on Siever 1983).

These laws, which for centuries provided the basis for geological mapping and stratigraphic correlation, no doubt have their limitations. For example, ripple migration on a sediment bed may cause the original layers to be deposited at an angle to the horizontal plane. Similarly, variations in depositional condition at the time of sedimentation (facies change) can interrupt the lateral continuity of sediment layers. Surprisingly, Steno's fundamental laws did not find practical application for nearly a century. Johann Gottlob Lehmann is believed to be one of the flrst to have applied the Law of Superposition to large-scale geological mapping in parts of Germany in the mid-18th Century. In 1815, a British civil engineer, William Smith (1769-1839), produced the fust stratigraphic section from the records maintained at the construction site. His field studies also led to the production of the fIrst geological map of Britain. In 1785, James Hutton (1726-1797), a Scottish physician, recognized the cycle of weathering, erosion and transportation by running water (see Fig. 1.1). He also observed

Introduction

3

that after consolidation the sediments laid down in the sea produce stratified deposits. According to many, Hutton's appreciation of the immensity of geological time and his rejection of supernaturalism in providing explanations for geological phenomena marked the beginning of rational thinking in geology. Hutton's ideas were publicised during his lifetime by his friend, John Playfair, and nearly half a century later, by Charles Lyell (1797 - 1875). Lyell, who expanded and modified the Huttonian thesis, was quick to recognise that the changes that have taken place in the geological past can be explained by the processes that are taking place today. In other words, the laws of nature guiding the earth processes have remained invariant, although the earth itself has changed with time. This concept of uniformity in the earth processes came to be known as the Principle of Un ifonn itarian ism. With time, many corollaries of uniformitarianism like 'the present is a key to the past' (a phrase coined by Geikie in 1882), developed from the original HuttonianLyellian percept. Some of these, such as 'the rates of the earth processes have remained constant', or that uniformitarianism should be called 'actualism' because it refers to actual or real events, have been challenged by later workers on the ground that they are fallacious. In spite of these criticisms it must be remembered that the HuttonianLyellian concept has been the basis for most of our geological thinking. Many of the so called fallacious concepts developed from the beliefs of later workers and can hardly be traced to the original writings of James Hutton or Charles Lyell. James Hutton was dedicated to the 'volcanist' idea that the internal heat of the earth is wholly responsible for the earth processes. This was in marked contrast to the 'Neptunist' concept, patronized by Abraham Gottlob Werner (1749 - 1817), that all rocks except recent volcanic lavas are produced from chemical or mechanical deposits in a universal ocean. So profound was the influence of the latter school on contemporary geologists that the idea of lateral change in depositional events (facies change), introduced by Gressly in 1833, did not find easy acceptance either in Europe or in North America for a long time. Geologists were still under the spell of the Neptunist belief that all rock layers initially laid down in a universal ocean, must also be continuous, without change. An important event in mid-19th century was the observation by James Hall in the Appalachian mountains of North America that the thickest sediments accumulate in long, linear belts of subsidence in the earth's crust. This observation eventually led to the formulation of the concept of geosyncline, a model that dominated the sedimentological world for nearly a century. Developments Since Late 19th Century The late nineteenth and early twentieth centuries witnessed rapid developments. Studies by pioneers such as Henry Clifton Sorby (1826 - 1908) in England led to the foundation of sedimentology as we know it today. Sorby was not only interested in the study of sedimentary structures and their implications, but was also one of the first to make extensive use of the petrographic microscope in the study of sedimentary rocks. He produced two classical volumes on sedimentary petrography and also a monograph on cheritified limestones. A few years later, another important study on lithogenesis, by Johannes Walther, appeared in Germany. Walther also formulated the Law of Facies Succession, which still bears his name (see Chapter 9).

4


The first half of the twentieth century witnessed the initiation of specialised studies in many different branches of sedimentology. Systematic studies on the processes of sedimentation and their bearing on stratigraphy were initiated in North America by Grabau, Barrell and Schuchert. Studies by W.M. Davis (1850 - 1934) and G.K. Gilbert (1843 - 1918) and later by Twenhofe1 (1875 - 1957), established a close link between surface processes of the earth, sedimentation and stratigraphy. This marked the beginning of the use of the present as a key to the past. In 1944 sedimentologists' interests in modern sediments were revived with the publication of studies on the Mississippi delta by H.N. Fisk and his associates. Classical studies on modern eolian deposits (Bagnold 1941; McKee 1979), modern fluvial deposits (Hjulstrom 1935; Sundborg 1956; Leopold, Wolman and Miller 1964), and modern tidal sediments (Reineck since 1952, see Reineck and Singh, 1980; van Straaten 1954, 1954a; Ginsberg 1975) have been emulated by sedimentologists throughout the world for nearly half a century now (see Chapter 7). Petrographic studies initiated by Sorby in England and Walther in Germany were continued, among others, by Grabau, Krynine and their students in North America (Chapter 3). The basic ideas on textural parameters of sediments were formulated during the first three decades of the present century (Udden 1914; Wentworth 1922; Wadell 1932, 1935; Zingg 1935; see Chapter 4). This was also the time when many of the procedures for sedimentological analyses were standardised. The trend of laboratory simulation of sedimentary processes, initiated in France by Daubree around 1870, was revived by G.K. Gilbert (1914) in North America. Since that time many of the hydraulic laboratories engaged in experimental studies on sedimentary processes have contributed greatly to our understanding of the basic processes involved in sedimentation. That such studies can be of importance to the perception of many fundamental geological problems was demonstrated in the Netherlands by Kuenen and Migliorini (1950). This was followed by many more contributions to experimental sedimentology throughout the world (see Chapter 5). Vant Hoff's experiments on marine carbonates in the early years of this century and Correns' experimental studies around 1920 in Germany, marked the beginning of sedimentary geochemistry. This field of study is presently actively pursued, particularly in North America (see Kiumbein and Garrels 1952; Garrels and MacKenzie 1971). A major event of the present century was the revival of interest in palaeocurrent research, initiated in the nineteenth century in England by He. Sorby. Palaeocurrent studies by Brinkmann and also by Cloos in Germany around 1930 were rejuvenated in North America (see Pettijohn 1962), to be followed by development of many sophisticated techniques in palaeocurrent research (see Chapter 10). The studies on tectonics and sedimentation, initiated in North America by James Hall around 1850, were pursued by Shvetsov in Russia, Bertrand in France and many others throughout the continent. Oceanographic expeditions, mainly by the Dutch sedimentologists and geophysicists in the East Indies provided valuable input for refinement of the geosynclinal model - ideas which were pursued and elaborated in North America (see Kay 1951). These ideas, together with the classical facies concept, which found aceeptanee in North America around 1930 and the petrographic studies initiated by Kiynine 1948, provided the background for much of what was achieved during the first half of the twentieth century.

Introduction

5

Recent Trends in Sedimentology

Developments until about the first half of the 20th century were mainly the outcome of individual efforts. The end of the Second World War saw a major change in this trend. Large-scale oceanographic expeditions by the major nations in the post-war period, development and deployment of submersibles, deep-toe equipment, side-scan sonar and undersea TV camera in subsea exploration, large-scale drilling operations at sea and refinement of core recovery techniques since 1950, have provided information, which was heretofore unobtainable. Much of what is known today about the siliciclastic and carbonate deposits of shallow seas has been revealed by these explorations. Discovery of giant petroleum reserves in carbonate rocks provided impetus for the study of carbonate petrography and carbonate depositional environments. Intensive research on carbonates, initiated by the major oil companies, led to the discovery of what is known today about carbonate petrography and facies. An equally intensive investigation of the present day carbonate depositional environments in. the Bahamas, Florida and the Persian Gulf also took place. Exploration for petroleum opened up new dimensions in organic geochemistry also. Techniques were developed for working out the maturation and thermal history of sedimentary basins. Deployment of sophisticated equipment such as X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscope (SEM), electron probe micro analysis (EPMA), cathodo-luminescence (CL) made investigation of fine-grained sediments and sedimentary rocks with a high degree of precision possible. Isotope studies (both stable and radio isotopes) provided clues to environmental interpretation. Application of mathematical and statistical techniques led to the investigation of subtle interdependence between lithological units and chemical constituents within stratigraphic sequences. The ideas developed during the last few decades have been revolutionary in many ways. Information available from hundreds of thousands of boreholes drilled in search of hydrocarbon has diverted the attention of sedimento1ogists to the subsurface. Techniques have been developed for recognition of sedimentary facies from well logs, cores and well cuttings. Developments in the field of trace fossil study (ichnology) now allow interpretation of depositional environment from information which, until recently, was considered insufficient (see Chapter 6). The current emphasis is on facies modeling from both surface and subsurface data. Developments in the field of seismic exploration have introduced concepts such as seismic and sequence stratigraphy (see Chapter 9). New concepts such as allostratigraphy and autostratigraphy have also been introduced. Simultaneously, the traditional ideas on basin development are being modified from the point of view of plate tectonics. Source Albritton 1963 ; Dunbar and Rodgers 1957 ; Gilluly, Waters and Woodford 1975; Hubbert 1967; Mather and Mason 1939; Mclntyre 1963; Miall 1978; Middleton 1973, 1978; Nelson 1985; Shea 1982; Zenger 1986.

LITERATURE ON SEDIMENTOLOGY In the early part of the 20th century there were hardly any text and reference books on sedimentology available. Since 1950 the figure has been increasing almost exponentially.


6

Table I. I lists the more important text and reference books on sedimentology under three heads: I) those dealing with general aspects of sedimentary processes and products, 2) those dealing with specialised sedimentological topics, and 3) those dealing with methodology for sedimentological research. All the texts mentioned in this table are written in English. Needless to say, many other well-written texts on sedimentology exist, particularly in the various European languages. Table 1.1: Important text and reference works on sedimentology

1910 Hatch & Rastall (1913) 1920 Milner (1922)

Twenhofel (1928) 1930 Twenhofel (1932)

Krumbein & Pettijohn (1938)

Kuenen (1950)

Carozzi (1960)

1940 Shrock (1948) Pettijohn, 1st ed. (1949) Twenhofel (1950) 1950 Krumbein & Sloss, 1st ed. (1951) Dunbar & Rodgers (1957) Pettijohn, 2nd ed. (1957) 1960

Milner, 2 vols. (1962) Potter & Pettijohn, 1st ed. (1963)

Krumbein & Sloss, 2nd ed. (1963) Griffiths (1967) Folk (1968) Bouma (1969)

Kukal (1970)

1970

Garrels & MacKenzie (1971) Carver (1971) Pettijohn, Potter & Siever (1973), Milliman (1974) Bathurst (1975), Wilson (1975) Potter & Pettijohn, 2nd ed. (1977) Reading, pt ed. (1978) Walker, 1st ed. ( 1978)

Blatt, Middletion & Murray (1972) Reineck & Singh 1st ed. (1973) Pettijohn, 3rd ed. (1975) Selley (1976) Friedman & Sanders (1978) Reineck & Singh 2nd ed. (1980)

Miall (1984) Walker, 2nd ed. (1984) Reading, 2 nd ed. (1986) Shannon & Naylor (1989) Miall, 2nd ed. (1989) Tucker el at. (1990) Syvitski (1991) Reading, yd ed. 1996

Galloway & Hobday (1983)

Texts on specialized sedimentological topics

1980

Lindholm (1987) 1990 McLane (1995) Leeder (1999) Nichols (1999) Texts on sedimentological techniques

Texts on general aspects of sedimentary processes and products

Introduction

7

The results of current research on sedimentological topics are available in periodicals.

Of these, the following three, published in the English language, are devoted wholly to sedimentology: Journal of Sedimentary Petrology (now called Journal of Sedimentary Research) published since 1931 by the Society of Economic Paleontologists and Mineralogists (renamed in 1989 as the SEPM Society for Sedimentary Geology), Tulsa, Oklahoma, USA; Sedimentology, the Journal of the International Association of Sedimentologists (LA.S.), published since 1962, and Sedimentary Geology (published since 1967 by Elsevier Publishing Company, Amsterdam). These three journals together publish more than 200 large research papers on sedimentological topics annually. Sedimentological research papers are also regularly published by many other geological journals. Notable among them, in the English language, are Bulletin of the American Association of Petroleum Geologists (A.A.P.G.), Bulletin of the Geological Society of America, Marine Geology (Elsevier, Amsterdam), Journal of Geology (Chicago), Geological Journal (Wiley), Lithos (Oslo), Journal of Petroleum Science and Engineering (Elsevier), Palaeogeography, Palaeoclimatology, Palaeoecology (Elsevier), Basin Research (Blackwell), Canadian Journal of Earth Science (Ottawa) and Geology Today (Blackwell). Several memoirs, special publications, short course notes, reprint series of A.A.P'G., S.E.P.M., and I.A.S. and the Developments in Sedimentology Series of Elsevier are devoted wholly to sedimentological topics. The Lecture Notes Series (Springer) are also often of interest to sedimentologists. Except for the publications of the Indian Association of Sedimentologists there is no Indian journal devoted wholly to sedimentology. All the Indian geological journals, however, frequently publish articles on sedimentology. Notable among them are: Indian Journal of Geology (formerly, Quarterly Journal of the Geological, Mining and Metallurgical Society of India) published from Kolkata (Calcutta) for over 70 years; Journal of the Geological Society of India (published from Bangalore since 1960) and Indian Journal of Earth Sciences (pubJIlshed since 1974 from Kolkata).

2 PROCESSES OF SEDIMENTATION

SURFACE PROCESSES AND ROCK WEATHERING

Sedimentary processes are largely controlled by just two agencies, water and wind. Thus the hydrological and wind cycles play very important roles in sedimentation. The hydrological cycle is schematically represented in Fig. 2.1. About two-thirds of the precipitation that takes place on land finds its way back to the atmosphere through evaporation and transpiration. The rest flows back to the sea mostly as surface run-off through streams and partly as ground water. The actual amount of water involved in the different phases of movement varies from place to place, depending on local geography and climate but by and large a balance in the water budget is maintained. Snow I, I

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" (Z)

Clouds

1i

!!

Evaporation & Transpiration

Land

Fig. 2.1: Schematic representation of the hydrological cycle. The balance between evaporation and precipitation in an ideal tropical region is indicated by arrows.

Water is also responsible to a large extent for weathering and decomposition of preexisting igneous, metamorphic, and sedimentary rocks. The actual amount of sediment yield depends not only on precipitation, but also on surface conditions, types of rocks exposed and density of vegetation cover. Rock weathering can take place by mechanical, chemical and biological actions. Freezing of water percolating into fractures andjoints of rock bodies causes expansion and disintegration in cold climate (frost heaving), because water expands when it freezes. Joints within rock bodies allowing free access of water play important role in this context. Alternate heating and cooling during day B

Processes of Sedimentation

9

and night, and growth of plant roots may also cause physical movement of the joint blocks. Earthquakes, the heat of lightning and the impact between large bodies of displaced rock masses are some of the other causes of mechanical disaggregation. In drier climate, crystallization of soluble salts within cracks and fissures can disrupt rock bodies. Mechanical abrasion and deflation are important erosive processes in deserts. Much of the actual disintegration of rocks takes place by chemical weathering. The process is particularly effective in tropical, humid climate where rainy seasons follow the summer months. Dissolved carbon dioxide ionizes rain-water into carbonic acid. Rain-water therefore, is generally acidic, with the pH ranging between 4 and '7. Lightning discharges and industrial contamination, particularly sulphurous gases, also produce some acids although in small quantities. The acidic water can take much of the natural carbonate into solution. Humus in the soil also makes natural limestones unstable. Oxidation of organic material in soils also releases CO 2, thereby producing an acidic microenvironment with a pH as low as 2. Chemical weathering of silicate minerals is a much more complex process. In carbonated water, feldspars break down into clay minerals, silicic acid and carbonates (Holmes and Holmes 1978) : 6H 20 + CO 2 + 2KAISiP8 = AI 2Si 20s(OH)4 + 4SiO(OH)2 + K 2C0 3 orthoclase kaolinite silicic' acid' Similarly, albite also produces kaolinite: 2NaAISi3 0 g + 2C0 2 + IlH 20 = AI 2Si20s(OH)4 + 2Na + 2HC0 3 + 4H 4Si04 albite kaolinite silicic acid Kaolinite, in fact, is often the first mineral to form as a product of chemical weathering. Depending on the chemical composition of the source rock being weathered, clay minerals such as smectite, illite, chlorite, and other oxides and hydrates may be produced. Pyrophyllite may be produced by breaking down of plagioclase. Much of the soda generated by the process of chemical weathering is transported to the sea by running water, but potash remains in the soil to be absorbed later either by plant roots or by clay minerals. The average dissolved silica content of the world's rivers is about 13 ppm. Much of this silica possibly comes from feldspar rather than quartz. Solubility of silica increases with an increase in temperature and alkalinity of the medium. With a decrease in temperature and alkalinity, much of the dissolved silica precipitates in the intergranular space of sediments to provide bondage between the grains. The trend of decomposition of the ferromagnesian minerals can be illustrated by the following example: H2 0 + CO 2+ Ca(Mg.Fe) (Si0 3}z ~ 2SiO(OH)2 + soluble bicarbonates of Ca, Mg, Fe diopside silicic' acid' Similarly, chemical decomposition of biotite, hornblende and augite leads to the formation of clay and chloritic minerals. Bicarbonate of iron Fe(HC0 3)2 is converted to limonite Fe 20 3 .H 20 in the ground-water zone on coming into contact with oxygenated water (see Fig. 2.2A). Ferrous iron released from minerals during weathering

10

Introduction 10 Sedimentology

is oxidized in soil to a ferric state. Common hydrated iron oxide minerals include limonite (Fe 20 3 .H 20), goethite (Feo.OH), lepidocrocite (FeOOR) and anhydrous dimorphs of hematite (n.Fe203)' These minerals, very stable under oxygenated condition, continue to exist in the soil almost indefinitely (Fig.2.2B). Highly-colourful, they impart yellow, orange or brownish red tones to the weathered soil surface. Some of these minerals also provide bondage between loose grains of sediments as ferruginous cement. The product of weathering of rocks is called regolith. It is constituted of mineral particles, and is the main source of sediments. Regolith produced in situ is called residual regolith. It overlies the bed rock from which it is derived. Residual regolith, in turn, is overlain by surface soil. Regolith may also be transported from a distant source and deposited on the bed rock downslope, or on an older layer of regolith. The medium of transportation may be water, wind or ice. Transported regolith that accumulates at the foot of a slope is called colluvium. The regolith transported further downwards and deposited in the valley is called alluvium. The main agency for transportation of alluvium is the stream (Fig. 2.3). Downslope movement of regolith, soil and hard rock under the influence of gravity is broadly designated as mass wasting. The process is given different names depending on the materials involved and their speed of movement. Talus creep involves rock bodies moving very slowly downslope. When the rocks move very fast they may cause landslides. Rock bodies moving still faster cause rockfall. Near the source regolith moves very slowly as soil creep and solifluction. When, aided by water, they move faster, they cause mudflows. Very fast moving regolith avalanches downslope. Most of the sediments produced on the exposed surface of the earth find their way to the sea through rivers. Chemical analysis of sea-water, however, does not exactly match the composition of average river-water. This is because some of the chemicals arriving in the ocean are precipitated there. The time during which an element stays in the sea-water (called residence time) can be obtained by dividing the amount of an element present in the ocean by the amount of that element (tons/year) contributed by rivers to the sea. Sea-water contains large proportions of sodium chloride because residence time of Na+ and CI- are of the order of 100 m.y. Since the residence times of Ca++ and Mg++ are much smaller (I m.y. and 10 m.y. respectively), these elements are absorbed by marine organisms such as corals to build up their shells and tests. These shells eventually produce extensive coral colonies. In the deeper parts of the ocean, where the temperature is low, pressure is high and the water is acidic due to dissolved gases, the carbonate is dissolved. This condition is likewise suitable for precipitation of dissolved silica, which has an extremely small residence time. Silica precipitation in deep ocean-waters in the form of chert is aided by microscopic marine organisms such as radiolaria and diatoms. Much of the deep ocean bottom is covered by siliceous ooze and also by extremely fine-grained, muddy sediments derived from land. It must be remembered that chemical precipitation is not confined to oceanic areas only. Natural springs, playas and caverns are some areas on land where chemical precipitation takes place.

MINERAL STABILITY The process of weathering effects considerable change in source minerals of s~diments. The degree to which a mineral can resist this change determines its stability.

®

J,

J,

2

O

J,

J,

°2 ~

.. ' FeO H 20 ......... 2 3

Fe(HC03)2

'----'"

J,

~

. LImonite

J,

Eh

-0.5

0

+0.5

2

4

6

\

.

®

8

\

10

Fe20 3 + H 20

,, ,

pH

SOl L

,

Fig. 2.2: (A) Diagammatic representation of the pocess of limonitization by oxidation of biocarbonate of iron in the ground-water zone (modified after Borchert 1960). (B) Stability fields of iron compounds under surface soil condition (partly redrawn from Garrels and Christ 1965).

J,

Rain water

12

......

......

:J

0'

:J

(I)

-a

3'

a.

(I)

( J)

o

-

Ul

(I)

Ul

!II

(I)

'lJ

oo


12

... + Fig. 2.3: Generation of regolith from weathering of bed rock (based on Strahler and Strahler 2002).

Interestingly, the stability series of rock forming silicates shows a trend almost opposite to that of crystallization as indicated by Bowen's reaction relation series (Table 2.1). For example, olivine, the earliest to crystallize from a melt, is also the first to be altered during chemical weathering. Quartz, the mineral last to crystallize, can survive chemical decay almost indefinitely (G-oldich 1938). Table 2.1 : Relative stability of common rock forming silicates (modified after Goldich 1938)

Most stable

Least stable

Quartz (200) Muscovite (170) Hornblende (160) Biotite (150) Augite (150) Olivine (133)

K-Feldspar Alkalic plagioclase (150-160) Calcic plagioclase

N.B. Figures in parentheses indicate the proportion of anion to a cation content of 100 in each case.

The explanation of this lies in the anion to cation ratios of the minerals. The proportions of negative ions (anion) for the common rock-forming silicates are shown in parentheses in Table 2.1, assuming that the cation cantent is 100 in each case. The higher the anion proportion of a mineral, the lower its susceptibility to chemical weathering, which essentially is a process of oxidation or hydration (introduction of 0 2- or OH-). Thus olivine with a relative anion content of 133 will be the fIrst to be affected by chemical weathering, whereas quartz, with a relative anion content of 200, will not change at all. Under this condition the process of weflthering leads to the production of increasingly stable minerals. Two common examples of this process are: 1. olivine (133) ~ chlorite (180) or serpentine (180) -t limonite (200). 2. feldspar (160) ~ hydromica (200) -t clay minerals (225) ~ bauxite (250 - 300).

(figures in parentheses indicate proportions of anions relative to a cation content of 100 in each case). The actual amount of chemical alteration depends not only on the chemical composition and crystal structure of a mineral, but also on local climate, mainly temperature and humidity, as well as the chemistry of the pore fluid.


13

The problem of mineral stability can also be studied from the point of view of thermodynamics. In a chemical reaction the sum of free energies of formation of all the reaction products minus the free energies of the reactants measures the free energy change of the reaction. The more negative the value of free energy change of a mineral, the more unstable it is. Hence its chances of reaction to produce a more stable product are also higher. In the stability series obtained by Curtis (1976), according to the above mentioned principle olivine (most negative) will alter first and muscovite (least negative) will resist alteration for a long time. The common rock-forming silicates, arranged in order of decreasingly negative free energy, are as follows: olivine, pyroxene, amphibole, plagioclase, microcline, muscovite. The trend is the same as that obtained by Goldich (1938, see Table 2.1). While free energy indicates the susceptibility of a mineral to chemical reaction, the physical stability of a mineral can be expressed in terms of its resistance to abrasion. The 'durability' of a mineral has been experimentally shown to be related to its hardness. The scale of resistance to abrasion, as worked out by Thiel (1945), for example, is as follows (starting with the mineral having least resistance) : barite, siderite, fluorite, goethite, enstatite, kyanite, hematite, augite, apatite, hypersthene, rutile, hornblende, zircon, epidote, garnet, staurolite, microciine, tourmaline, quartz. Many minerals in this list are' heavy minerals', that is, they have a specific gravity higher than that of quartz (p =2.65) or feldspar (p =2.54 - 2.76). For practical purposes, all minerals having a specific gravity higher than that of bromoform (p =2.87 at 20° C) are listed as heavy minerals. According to stability the heavy minerals may be grouped as follows (modified by Pettijohn 1975, after Sindowski 1949) : Most stable: zircon, rutile, tourmaline. Intermediate: staurolite, kyanite, hornblende. Least stable: (garnet), augite, apatite, olivine. Because of their stability, zircon, rutile and tourmaline survive prolonged weathering and concentrate in the older soil profiles. Garnet is generally considered to be more stable than that shown here. SOURCE OF SEDIMENTS The sediments derived directly from crystalline igneous and metamorphic rocks are the first-cycle sediments. Sediments produced out of lithified first-cycle sediments are termed reworked (polycyclic) sediments (see Fig. 1.1). Reworking can be detected from textural evidence. During the process of cementation of a first-cycle quartz sand precipitation of silica on detrital quartz grains may produce overgrowths. If the edges and corners of the overgrowth on a quartz grain of first-cycle origin are found to be well rounded, we know that the grain of quartz is of second-cycle origin. The source area and the source rock of a sediment and also, according to some authors, the climate, relief and the environment prevailing at the source, are collectively called its provenance (French:provenier meaning 'to originate or to come from'). The mineralogical constituents of terrigenous sediments bear the imprint of the source rock and also, to some extent, its climate and relief. Quartz, feldspar and heavy minerals are particularly suitable for deciphering provenance.

14


Quartz and Feldspar as Provenance Indicators Quartz, the most abundant derital constituent in sedimentary rocks, is often used as an indicator of provenance, but opinion differs about the specific characters of a quartz grain which are indicative of its origin. Undulatory extinction (,strain shadows'), traditionally taken to be indicative of a metamorphic source, has been found in quartz grains of plutonic origin also (Blatt and Christie 1963), Acicular inclusions, long believed to be a characteristic of quartz of granites, have also been detected in metamorphic rocks. No significant difference between shapes of quartz grains of igneous and metamorphic origin have been statistically established, although this was, at one time, believed to be an important discriminating criterion. A group of geologists at Indiana University, after ,a thorough review of the problem, came to the conclusion that strongly undulose and finely polycrystalline quartz grains are indicative of metamorphic source rocks, but the quartz grains of plutonic and volcanic origin are characteristically strain free (Basu 1985). The Indiana group developed a method of discriminating sedimentary rocks of plutonic, low- and high-rank metamorphic parentage by plotting the proportions of the following four parameters in a 'diamond diagram' : undulatory quartz, nonundulatory quartz, polycrystalline quartz, and the number of crystal units per polycrystalline grains (Fig. 2.4). As it is very stable, quartz survives prolonged weathering and erosion. The ratio of quartz to feldspar (plus unstable rock fragments) can therefore be used as an index of mineralogical maturity of a sediment of mechanical origin. Comparative studies of sand populations transported under various climatic and environmental regimes have shown that a unique combination of extreme climate, relief, transportation, and rate of sedimentation is necessary for elimination of all the unstable fractions leading to a highly matured, frrst-cyc1e quartz sand. This imprint of climate, although preserved for the first 75 km of transportation in high-gradient streams, is rapidly destroyed as soon as a high-energy marine environment is reached. For this reason, palaeoclimatic interpretation from mineralogical composition is feasible only for terrigenous, firstcycle sand (Suttner et al., 1981). Feldspar, which ranks only next to quartz in abundance, may also act as an indicator of provenance. The chemical composition of feldspar may be useful for this purpose. Homogeneous alkali feldspars more sodic than Ab so are generally of volcanic origin. Alkalic feldspars more potassic than Or88 are derived from plutonic and m~tamorphic sources (Trevena and Nash 1981). Iden tifica tion of the structural states of detrital alkali feldspars may also lead to recognition of the parent rock. Detrital feldspars from metamorphic sources are mostly microc1ine, while those from volcanic ~ources are generally sanidine. Alkali feldspars with relatively higher Al!Si structural states are likely to be associated with rapidly cooling outer shell of a pluton rather than its core. This change in the degree of ordering of feldspar is reflected in the stratigraphic sequence produced out of gradual unroofmg of batholiths that have served as a source of sediments (Basll 1977} A quirk method for tne determinatiDn of the structural states of alkali feldspars for this purpose was developed by Basu and Suttner (1975). In the modern big rivers of the world the proportion of quartz to feldspar increases when the climate is tropical to subtropical, suggesting a lesser chance of survival of


15

Polycrystalline quartz (2-3 crystal units per grain; ~ 75%) of total polycrystalline quartz

I.

..

vI

I

• '" c

I

t;O

...

:

~o·""I.·.

0, .

t::::lo.el •

t

......

Non undulatory quartz

J~t'::::;/" .. :0.' ..... ~ ,

.
.0

_,

.•

....

....

0

tf.~1 .......~ {:f. I •

0'::: 8+'-+--+----1f--+--4 Undulatory ~ v • "fj ,$ / ij ~ quartz .!j I ~ f!. ~_f I . s § "0 IJ.J

....'

I

,-,;'.0'll'

'. / •• "'.$

• • i :' I

~

0

• Chlorite and biotite zone o Garnet to sillimanite zone • Granulite zone " Granitic plutonic (open symbols indicate anomalous samples

Polycrystalline quartz (> 3 crystal units per grain; > 25% of total polycrystalline quartz) Fig. 2.4: Diamond diagram showing distribution of quartz grains of different varieties with respect to plutonic igneous and metamorphic sources (after Basu et al.• 1975). With permission of SEPM (Society for Sedimentary Geology).

feldspar in humid climate (Potter 1978, 1978a). The ratio of quartz to feldspar in the fIrst cycle fluvial sand is > I in the humid Appalachians and < 1 in the relatively arid Rocky mountains of the United States (Basu 1976). The foregoing suggests the lesser chance of survival of feldspar in humid climate but it may not be altogether impossible for feldspar to survive prolonged transportation in humid climate although its size decreases by cleaving. The latter conclusion is reached from the finding that irrespective of climate, the proportion of feldspar increases with a decrease in grain-size. A certain amount of climatic rigour, either arid or cold, was long believed essential for the preservation of fresh feldspar grains in sediments, but a later discovery of fresh feldspar under humid, tropical, South American climate led to the belief that short-distance transportation followed by quick burial is the key to the preservation of fresh feldspars in nature (Pettijohn 1975). This implies sudden uplift of the source area and quick

16


subsidence of the basin of accumulation, but the effects of climate and tectonism need not be mutually exclusive in the preservation of fresh feldspars in sediments. According to current thinking, the proportion of quartz and the relatively unstable elements such as feldspar and rock fragments in a sediment population is dependent not only on the source rock and climate, but also to some extent on the degree of crustal stability (Dickinson and Suczek 1979). These ideas will be discussed later, under tectonics and sedimentation (Chapter 8).

Heavy Minerals as Provenance Indicator While minerals such as quartz and feldspar can be derived from a variety of rocks, the heavy minerals, or more particularly, assemblages of certain suites of heavy minerals, are restricted to some distinctive source rocks only. Heavy mineral assemblages identified with major source rocks, igneous, sedimentary, and metamorphic, are shown in Table 2.2. Table 2.2: Heavy mineral assemblages of the major source rocks (condensed from FeoCodecldo 1956)

Heavy mineral assemblage

Source rock

Apatite, biotite, hornblende, monazite, muscovite, rutile, pink tourmaline, zircon

Acid igneous rocks

Fluorite, garnet, muscovite, topaz, blue tourmaline, monazite

Granite pegmatites

Augite, diopside, hypersthene, magnetite, olivine, chromite

Basic igneous rocks

Andalusite, garnet, staurolite, zoisite, epidote kyanite, sillimanite

Metamorphic rocks

Barite, iron ores, rutile, tourmaline, zircon (rounded grains common)

Reworked sediments

(~

In vertical stratigraphic columns heavy minerals often occur in a reverse order of stability. For example, tourmaline, zircon and rutile, the most stable of the heavy minerals, occur in the older beds, whereas much less stable amphibole, pyroxene and. olivine are often found in the younger horizons. As an explanation of this mode of occurrence it has been suggested that the older beds have lost the less stable minerals due to the action of the circulating ground-water or interstratal solutions which have altered or removed these minerals from the rock. If this is actually the case, they can hardly be used as indicators of provenance (Pettijohn 1975, p. 499).

3 SEDIMENTARY PETROLOGY

After transportation and deposition loose sediments are consolidated into sedimentary rocks by a process called diagenesis. The sedimentary rocks thus formed may be broadly divided into two groups: exogenetic and endogenetic. Exogenetic rocks are the products of fragmentation of the source rock outside the basin of sedimentation. Endogenetic rocks are produced out of precipitation from solution within the basin. The exogenetic and endogenetic rocks can be distinguished by their textures. While the endogenetic rocks show an interlocking or crystalline fabric (,non-clastic' texture), the exogenetic rocks show discrete framework grains separated by voids. The latter texture is called 'e/astic' from the Greek clastos, meaning broken. Classification of sedimentary rocks into clastic and non-clastic, though popular lmong sedimentologists, may at times lead to ambiguity. A chemically precipitated ;ediment for example, may also be transported from its place of origin after ragmentation and deposited elsewhere to form a sedimentary rock. Such rocks, llthough essentially endogenetic, may show a clastic texture. To avoid confusion in ouch cases, Folk (1968) introduced the following terms: terrigenous, orthochemical, and lliochemical. Terrigenous rocks are produced from sediments derived from land areas located mtside the basin. The process of derivation is purely mechanical but not all terrigenous :ediments are the products of weathering and erosion. Fragments of volcanic origin md the products of crushing by glaciers also add to the terrigenous sediments. The )rthochemical or truly chemical rocks are produced by chemical or biochemical )recipitation within the basin. Evaporites such as halite, anhydrite and gypsum and lOn-evaporites such as limestone, dolomite and ironstone are examples of such >recipitates. Intermediate between the terrigenous and orthochemical rocks are the lllochemical rocks (allo means false). Produced initially within a basin by chemical )recipitation, these rocks are fragmented, transported and redeposited elsewhere. Both errigenous and allochemical rocks show a 'clastic' texture. Only the orthochemical -acks show a non-clastic texture. Sediments of chemical and terrigenous origin may :ombine to form rocks of a hybrid nature. Calcareous and carbonaceous shales are !xamples of such hybrid rocks. While accepting the rationality of Folk's scheme of :lassification, it must be appreciated that the terms clastic and non-clastic have horoughly pervaded sedimentological literature and cannot be totally ignored at )resent. These terms will be used in the present tex' whenever indispensable. 17

18


Of all the major groups of sedimentary rocks found on the earth, only three, sandstone, shale and limestone, together account for about 95% of the total. Theoretical calculations, based on the idea that the average chemical composition of the sedimentary rocks must match the average composition of the igneous rocks from which they are derived, lead to the following figures of abundance of the major rock types : shale (81%), sandstone (11%) and limestone (8%) (Garrels and Mackenzie 1971). Actual measurements of the stratigraphic record, however, show a smaller proportion of shale and a higher proportion of sandstone and limestone (Pettijohn 1975). ROCKS OF MECHANICAL ORIGIN Sedimentary particles of mechanical (terrigenous) origin are classified according to their size. Particles larger than 2 mm constitute gravel (Latin-rudite), those between 2 and 1/16 mm constitute sand (Latin-arenite), while those finer than 1/16 mm constitute silt and day (Latin-lutite). Accordingly, sedimentary rocks of mechanical origin are classified broadly into three groups: rudaceous, arenaceous and lutaceous (also called argillaceous). This scheme of classification is followed here for describing rocks of mechanical origin.

Rudaceous Sedimentary Rocks (Conglomerates and Breccias) Unconsolidated sediments coarser than 2 mm are grouped under the term gravel. Indurated gravels are called conglomerates. Since gravel is classified as granule, pebble, cobble and boulder (see Table 4.1)~ these terms are often used as prefixes .t:o indicate the size of the dominant fragments in a conglomerate (e.g., pebble conglomerate, boulder conglomerate). While the particles constituting a conglomerate are essentially rounded, a breccia consists of gravel-size clasts which are angular. The term rubble is applied to an assemblage of loose angular fragments of gravel size. Breccias need not be of sedimentary origin only. For example, fault breccias are produced by tectonic activity. Large, well-rounded particles of volcanic origin (volcanic bombs) constitute a rock. called an agglomerate. Like other sedimentary rocks of mechanical origin the space between the framework grains of a conglomerate is filled by finer particles. These fine particles constitute the matrix of the rock. Gravel and sand fragments can combine in various proportions to produce a conglomerate. The resultant rock is named accordingly. Conglomerates may be classified either in a purely descriptive way (example: chert conglomerate, limestone conglomerate) or according to the mode of their origin (fluvial conglomerate, glacial conglomerate). In the comprehensive genetic classification proposed by Pettijohn (1975), conglomerates are grouped into four major categories which are geologically meaningful: epiclastic, pyroclastic, cataclastic and meteoric (see Table 3.1). Epic/astic conglomerates are, the commonest of all conglomerates. These consist of gravels of terrigenous origin. The conglomerates of fluvial origin as also those generated on beaches by wave action are of this type. The intraformational conglomerates, which are also epiclastic, are composed of clasts derived from within the basin. Shale-pebbles, produced out of mud cracks, are good examples of intraformational conglomerates, but the process may be more complicated.

Sedimentary Petrology

21

marine to non-marine, through mixed (see Chapter 7), sand-size fragments of quartz, feldsprs, micas, heavy minerals and rock fragments constitute the bulk of framework grains of sandstone. Rocks composed of carbonate or pyroclastic grains are not treated as sandstones. The voids between the framework grains of sandstones are fllied either by finer grains of terrigenous origin or by chemically precipitated cement. Under a microscope the cement shows a crystalline texture but the matrix is composed of discrete, detrital particles. Matrix is a relative term. Particulate materials (irrespective of their actual sizes) distinctly smaller than the framework grains constitute the matrix of a terrigenous rock. In the case of a conglomerate, sand-size grains constitute the matrix, whereas in sandstones the matrix is composed of fine, mud-size grains only.

Classification of Sandstones Sandstones are classified by observable petrographic criteria. Proportions of quartz, feldspar, rock fragments, and mica are commonly used for this purpose. Being the most stable of all detrital grains, quartz survives prolonged weathering and abrasion at the source and also during transportation. A sandstone composed wholly of quartz grains is considered the most matured. For this reason the ratio of quartz to the relatively unstable components, feldspar and rock fragments, is used as an indicator of the mineralogical maturity of a sandstone. The proportion of feldspars and rock fragments is indicative of the nature of the source rock. When sand is transported in nature by running water, the finer fractions are carried in suspension while the coarser ones are moved along the bottom. The very fine grained clay is carried as colloidal suspensIon. The clay particles, being electrically charged, adhare to each other to increase the viscosity of the transporting fluid. Grain-to-grain interaction of even the non-electrically charged fine particles. might also increase the resisting stress within the fluid, thereby increasing its apparent viscosity (Bagnold 1956, see also Leeder 1999, p. 126). The finer fractions eventually contribute to the matrix of the sandstone. The fluidity of a running body of water being inversely proportional to its suspended load, it is argued that a large amount of matrix in a sandStone is indicative of low fluidity of the transporting agency. The matrix content of a sandstone is thus important for its interpretative value (Pettijohn 1975). This idea, however, has been challenged by some authors on the ground that a part or whole of the matrix may be of secondary origin. Increase of matrix content of many sandstones with increase of ageor with decrease of framework grain-size led to such a conclusion (Dickinson 1970; Okada 1971). Many different schemes of classification of sandstones have been proposed from time to time by various authors using the four criteria -quartz, feldspar, rock fragments, and matrix. The difficulty involved in plotting four variables on two-dimensional diagrams prompted some authors to use tWo separate triangles-one devoted to highmatrix sandstones (called wackes) and the other to low-matrix sandstones (called armites). In eacn case me ena memoers at the triangles are quartz, teldspar and rOCK tragmems (Gilbert 1954). Arkosic and lithic are the adjectives used for sandstones having high (> 25%) feldspathic and lithic contents respectively. While the schemes using two separate triangles for wackes and arenites have practical advantages, they divide sandstones into two rigid compartments. In reality, however, a sandstone may have any amount of matrix ranging from zero to about 30%. The idea

22


of a continuous change in the matrix content of sandstones was introduced by Dott (1964) who used a three-dimensional prism instead of two fixed triangles. Sandstones of various matrix content were represented by triangular sections of this prism. The boundary between arenites and wackes was arbitrarily placed by Dott at a matrix content of 15%. The end members of each triangle were quartz, feldspar, and rock fragments. This technique of representation is a closer simulation of the real situation than that proposed by other authors.

Fig. 3.2: Typical glaciated pebbles from the Talchir Boulder Bed, Giridih, India. Note pentagonal shapes and striated, pitted surfaces. Scale in cm.

Figure 3.3 gives a modified version of Dott's scheme of classification. The demarcation between quartz arenite and quartz wacke here has been put at 12.5% (or 1!8th of the field of view of the microscope, which is easier to estimate than 15%). The boundary between sandstone and mudstone has been put at 30% because a sandstone, by definition, must have about 70% sand-size grains. Quartz and metaquartzite fragments have been grouped together because they behave similarly from the point of view of mineralogical maturity. Chert grains have been included in rock fragments because chert is a distinct indicator of provenance (Okada 1971). For reasons to be discussed later, graywackes have been plotted in a separate triangle at 'the high matrix end of the prism. The daughter triangle of Fig. 3.3 gives a detailed classification of lithic arenites as orooosed bv Folk (1968).


23

Sandstones with appreciable quantities of slate, phyllite, mica schist and other phylloid fragments of low-grade metamorphism are termed phyllarenites. Those containing large proportions of detrital limestone fragments are called calclithites. Sandstones with chert or volcanic fragments are named chert and volcanic arenites respectively.

F~

______- L_ _ _ _ _ _ _ _

~

Fig: 3.3: A revised version of Dott's (1964) scheme of classification of sandstones. Q = quartz + metaquartzite, F::::: feldspars, R.F = rock fragments. The daughter triangle shows subdivision of lithic arenites after Folk (1968).

Description of Sandstones Quartz Arenite (a/so called Orthoquartzite) Quartz arenites consist almost entirely of quartz grains cemented together by silica or welded together by predissolved quartz (see diagenesis). Chemical precipitates other than silica may also serve as cement. The quartz grains constituting the framework are often well rounded and well sorted (Fig. 3.4). A small amount of heavy minerals of stable variety (tourmaline, zircon, rutile, ilmenite) may be present in quartz arenites. The Rewa sandstones of the Vindhyan Supergroup may be cited as an Indian example of quartz arenite. Modal analyses of representative samples of Rewa sandstones are presented in table 3.2. The detrital grains of quartz in arenites may be produced by erosion of vein quartz. They may also be derived from one of the following source rocks through prolonged weathering, erosion and transportation under high energy conditions (Folk 1968).


24

1. Plutonic (granitic) source ~ arkose ~ subarkose ~ quartz arenite. 2. Metamorphic source ~ graywacke ~ subgraywacke ~ quartz arenite. 3 . Sedimentary source ---+ submature orthoquartzite ~ mature orthoquartzIte (quartz arenite).

Fig. 3.4: Photomicrograph of a quartz arenite under plane polarized light. Upper Proterozoic Chandarpur Formation near Mahasamund, Raipur, India (thin section No. SCH3 , courtesy of S. S. Das and C. N. Rao) . Table 3.2 : Modal analyses of some Indian sandstones

Sample No.

Qllartz& Quartzite

Feldspars

Micas

Quartz Arenites 20 .4* 93.90 20.1* 88.53 20.10* 86.16 Quartz Wackes AM 18W+ 66.00 3 .00 8.00 AM 13+ 72.00 2 .00 3.00 AM 8+ 75 .00 2.00 2.00 Arkosic Wacke & Calcareous Arkose M 30 .. 52.5 7 25.11 3 ..14 45.57 20.05 M 46 .. 46.54 M 5 .. 20 .66 13.90

Rock Cement ]rag- (calcammts reous) 0.80 0.18 0.21

Cement Cement Matrix (ferru (siliceous) gino us) 3.45 5.83 3.04

2.00

Total

99.87 100.00 99.79

1.72 5.46 10.38 3.00 7.00 5.00

18.00 16.00 16.00

100.00 100.00 100.00

4.18

14.99 3.52 18.69

99.99 100.00 100.00

30.86 0.21

* Rewa sandston es, Vindhyan Supergroup (Precambrian), Mihar, India (Basumallick 1962). + Sandstones from Ironstone Shale Formation (Permian), Raniganj Coalfield, India (A. MandaI 1985, Unp u b. M .Sc thesis, lIT, Kharagpur) . .. Sa ndstones from Ra niganj Formation (Permian), Jharia Coalfield , India (S. Nayak 1985 Unpub. M .Sc thesis, lIT, Kharagpur).


25

Chemically, quartz arenites consist of a very large proportion (> 95%) of Si0 2 and small amounts of CaO, CO 2 , A1 20 3, FeO + Fe203' The chemical composition is controlled mainly by the type and proportion of heavy minerals and cementing materials in the rock (see Table 3.3).

Quartz Wacke Sandstones with a fairly large proportion (say> 12.5%) of fine-grained matrix are termed quartz wackes. The presence of a large proportion of matrix, a wide range of grain-size variation, lack of rounding, and sorting of the framework grains are indicative of textural immaturity. The presence of feldspars and rock fragments of various types indicates mineralogical immaturity. A fine-graine~ matrix is the dominant binding material in these rocks. A large amount of matrix in wackes leaves hardly any space for cement {Eg. 3.5). In some quartz wacke;, replace~~r.t cl the matrix by d~micaH:f precipitated cement may create the illusion of an arenite, but the textural immaturity of these sandstones is clearly indicated by lack of rounding and sorting of the framework grains . A high proportion of trapped detrital clay in the interstices of framework grains (protomatrix), reflects an abundance of suspension load in the transporting medium, as in a turbid river during flood , but a certain amount of matrix may also be produced at a later stage by diagenetic alteration of sand-size grains (epimatrix), or deformation and squashing of pelitic fragments (pseudomatrix) . Recrystallized material occurring between framework grains constitutes orthomatrix. The sandstones belonging to the Barakar, Ironstone Shale and Raniganj Formations of the Indian Gondwana are good examples of quartz wackes. Mineralogical compositions of ,ome of the typical quartz wackes are given in table 3.2.

Fig. 3.5: Photomicrograph of a quartz wacke under cross·polarizers . Nahan Group , Koshailia Nala , Kalka, India (thin section No. DP 25/6 8 , courtesy of D. P. Sen) .

26


Feldspathic Sandstone and Arkose A sandstone with a high (> 10%) proportion of feldspar is designated as a feldspathic sandsto ne. When the matrix content of such a rock is also high, the rock is called a feldspathic wacke. Low matrix feldspathic sandsto nes are classified as arkose or subarkose depending on whether the feldspar content is greater or less than 25% respectively. Arkose therefore is the name restricted to arkosic arenites with a high (> 25%) proportion of feldspar and a very low amount « 10%) of matrix (Fig. 3.6) . The term arkose should be used with caution. Many feldspar-bearing sandstones which seem to be arkose at first glance, turn out on detailed scrutiny to be subarkose or arkosic wacke.

Fig. 3.6: Photomicrograph of an arkose under cross-polarizers. Note large grains of feldspars. Barakar Formation, Central Godavari Valley, India (thin section No. SSG-1, author's collection).

Next to quartz and feldspar, the mineral which is frequently present in arkose is mica, both of the muscovite and biotite varieties (Table 3.2). Calcite is a common cementing material. Since arkoses are commonly derived from granitic sources, the proportion of K-feldspar always exceeds plagioclase. In chemical analysis this is reflected by a higher proportion of K2 0 and a lower proportion of Na 2 0 (see Table 3.3). The presence of large proportions of fresh grains of an unstable mineral such as feldspar in arkose is puzzling. For along time sedimentologists maintained that lack of moisture in the environment is essential for preservation of freshness of the feldspal grains. A warm-arid or cold-dry condition was suggested as the essential prerequisite. A later discovery of fresh feldspars in regoliths produced under humid, tropical conditions led to the conclusion that the key factor in the preservation of feldspar i~ tectonics, not climate. Quick burial without much transportation, due to sudden upheaval of the source and rapid subsidence of the area of deposition, may lead tc preservation of fresh feldspar grains. It should be remembered, however, that both climate and tectonism could have played important roles in the preservation of fresh feldspars in some sandstones.

27


Table 3.3: Average chemical composition of major sandstone types (data from Pettijohn 1975, Table 7.3; averaged by Potter, Maynard and Pryor 1980) Orthoquartdte (wt. %)

Si0 2 A1 20 3 Fe 203 FeO MgO CaO Na20

KzO

95.4 1.1

0.4 0.2 0.1 1.6

0.1 0.2

Graywacke (wt. %)

Arkose (lVt.%)

56.7 13.5

77.1

8.7

1.6

1.5

3.5 2.1 2.5 2.9 2.0

0.7 0.5 2.7 1.5

CO 2 C

1.1

1.2

2.8 2.8

HzO

0.3

0.1 3.0

0.9

Graywacke

Graywackes, which look very much like volcanic igneous rocks in hand specimens, belong to an exotic group of sandstones. Typical graywackes, as described by Krynine (1948), contain a medium- to fine-grained, angular framework of quartz, chert and rock fragments such as slate, schist, phyllite, set in large proportion of fine-grained matrix. Silt-size grains of illite, chlorite, micas, and various alteration products of basic igneous rocks constitute the matrix. Feldspars, abundant in graywackes derived from high-rank metamorphic sources, are usually of the sodic plagioclase variety. Graywackes, often found in association with a huge thickness of shales and radiolarian chert, are believed to be the product of sand carried into the deep sea by density (turbidity) currents although not all turbidites are graywackes (for example, see Okada 1966). Typical graywackes are commonly associated with altered basic' intrusives (,greenstones') and often grade into volcanic wackes. Most of these rocks have been reported from orogenic belts of Precambrian to Cenozoic age. The term greywacke was originally assigned by the 18th century miners in the Harz mountain in Germany to a rock with an exceptionally high matrix content (hence called grau + wacke, the German for gray, pasty matter; 'gray grit' or 'dark, dirty sand '; Krynine 1948). Since that time the term has been used for different types of rocks or even assemblages of rocks. The definition of graywackes became so ambiguous that any dark-coloured rock not positively known to be igneous came to be jestfully referred to as a greywacke in the Canadian Shield. For these reasons questions have been raised about the desirability of retaining the term greywacke. Tectonic or turbidite origin is implied in the classical usage of the term, but rocks similar to graywackes have been reported from fluvio-estuarine sediments also, while clean, non-graywacke sand has been found interbedded with argillaceous sediments of deep-water origin. Compositions of the type greywacke of the Harz mountain of Germany, and those of some arkose of France are known to overlap (Okada 1971). Considering all these, some authors recommend replacement of the term greywacke by a more general term, such as 'wacke' (Okada 1971; Friedman and Sanders 1978; Fritz and Moore 1988). In his threecomponent scheme of classification Okada (1971) recommend use of only two groups

28


of sandstones: arenites and wackes. The feldspathic and lithic wackes of this scheme encompass all sandstones, including those that hClve been traditionally designated as graywackes. The term greywacke and arkose have long pervaded sedimentological literature and discontinuation of these terms would merely add to the existing confusion. Moreover, graywackes, with their abundant matrix, and angular framework grains, create a distinctive textural pattern (see Figs. 3.7 and 3.8). It is proposed here to include these sandstones in a new triangular field at the high matrix end of the revised Dott diagram (Fig. 3.3). Consequently, the term greywacke has been remoyed from its usual place in the original Dott diagram of 1964.

Fig. 3.7: Photomicrograph of a graywacke, Chandpl,H Formation (Cambro-Ordovician?), north-west of Nainital. India . Plane polarized light. Note high proportior. of matrix (thin section No. N-1. author's collection).

The problem of the occurrence of a large proportion of matrix in graywackes was studied experimentally (Kuenen 1966, Kuenen and Sengupta 1970). More recent experimens (Ghosh, Mazumdar, Saha and Sengupta 1986) indicate that, contrary to common belief, a wide range of grain-sizes, from Coarse to fine, may be simultaneously deposited from a suspension when the flow velocity of a stream carrying a large amount of suspension load is decreased. The fine-grained particles deposited in the intergranular pores of the coarser framework grains might, in such cases, produce a texture similar to that of a grevwacke. Chemical Composition

oJ Sandstones

Average chemical compositions of orthoquartzites, arkoses and graywackes are shown in Table 3.3 . As expected, an orthoquartzite consists almost wholly of silica, whereas the proportion of alumina is quite high both in arkose and greywacke. The presence of

29


aluminosilicates, like feldspars, accounts for the alumina in these rocks. In arkose potash exceeds soda because of the presence of potash {eldspars but the proportion is often reversed in graywaekes due to the presence of sod ie plagioclase.

Fig. 3.8: Photomicrograph of a graywacke from Mt. Marion Formation (Middle Devonian), north-east of Monroe, USA. Note angula( framework grains 'floating' in a high proportion of matrix (Plane polarized light) . Author's collection . Table 3.4: Average bulk chemical composition of (:;ondwana sandstones from Raniganj basin, India (after Suttner and Dutta 198(;) Gondwana, India

North America LA (Blatt et al.1972) Si0 2

66.1 8.1 1.3 0.9 6.2 1.4 2.4

Alz03 KzO Nap CaO FeO MgO MnO Ti0 2

AR 77.1 8.7 2.8 1.5 2.7 0.7 0.5

PzO s

Ignition loss SiO/Al z0 3 LA VI IV II

8.3

8.7

II

IV

(n = 5)

(n = 7)

(n = II)

70.30 10.50 1.99 1.29 2.34 3.42 0.68 0.58 0.27 0.36 5.50 6.7

79.60 7.73 1.11

72.70 9 .31 2.25 1.20 3 .68 2.90 0.77 0.07 0.47 0 .26 4.18 8.2

I

O.Al j}3

1.0 1

OA9 0.03 0)9 0)5 4.51 10 .9

V

VI

=2)

(n = 4)

93.50 2.46 0.78 0.04 0.08 0.60 0.07 0.01 0 .11 0.50

92.15 3.12 0.35 0.02 0.05 1.31 0.06 0.02 0.48 0.35

38.4

29.5

(n

average lithic arenite; AR-average arkose· sandstones from upper part of Panchet Formation. sandstones from upper part of Barren M easures (,Ironstone Shale') Formation + Lower Panchet Formation. sandstones from Barakar and lower part of the Barren Measures (,Ironstone Shale ') Formation .


30

Chemical analyses of Indian sedimentary rocks are rare. The average chemical composition of a few Gondwana sandstones from the Raniganj coalfield, West Bengal, are reproduced in Table 3.4 from Suttner and Dutta (1986). The average chemical composition of lithic arenites (LA) and arkoses CAR) is also included in this table. In each of these analyses potash exceeds soda, indicating the presence of K-feldspars. The calcium found in these rocks is apparently derived from the calcium carbonate cement. Average modal analyses of the Gondwana sandstones from the Jharia and Raniganj basins have already been shown in Table 3.2. The feldspars of the sandstones belonging to the Ironstone Shale Formation of Raniganj coalfield account for high Al20 3 content of these rocks. The presence of appreciable quantities of FeO and CaO is obviously due to the ferruginous and carbonate cements. The chemical and mineralogical composition of graywackes may vary considerably, depending on their occurrence in relation to particular geotectonic settings. These are discussed in detail in Chapter 8.

Lutaceous (Argillaceous) Sedimentary Rocks Lutaceous sedimentary rocks are composed mainly of grains smaller than sand size. Sediments finer than sand but coarser than clay are classified as silt. Silt and clay together constitute mud. Indurated silt is called siltstone. Indurated, non-laminated mud forms mudstone. When mudstone is laminated and fissile, the rock is designated as shale. About 1/3rd to 2/3rd of such rocks are composed of clay -size particles, the rest being silt (Table 3.5). The fissility of shales is attributed to parallel arrangement of the constituent flaky minerals. Black shales, rich in organic matter, are particularly fissile while calcareous shales have no fissility at all. Table 3.5: Classification of indurated argillaceous rocks (more than 5{)% 01 grains are finer than sand). (Modified after Potter, Maynard and Pryor 1980)

Proportion of clay-size constituents (rest mostly silt)

0-32%

33-65%

66-100%

Beds> 10 mm thick

Bedded Siltstone

Mudstone

Claystone

Laminae < 10 mm thick

Laminated Siltstone

Mudstone (Shale)

Clay·shale

Shale constitute more than half of the stratigraphic record and hence are important. A study of shales is not as easy as that of sandstones, however, because the grain-size involved is very small. A study of the clay minerals which constitute a substantial part of all shales is possible only with advanced techniques. Shale chemistry is considered more important than shale petrology. Si0 2 and Al 20 3 together constitute the dominant part of common shales. H 2 0 and CO 2 constitute about 10%. Five other oxides, Fe203, Fe 0, K 2 0, MgO and CaO account for about 15-20% of common shales. Potash generally exceeds soda. In Table 3.6 the chemical compositions of some Indian shales of Precambrian and Permian age have been compared with that of the average shale composition.

31

Sedimentary Petrology Table 3.6: Chemical composition of shales

2 Si0 2 Al z0 3 FeZ03

FeO MgO CaO Na20

Kp

58.10 15.40 4.02

2.45 2.44 3.11 1.30 3.24

Ti0 2 MnO

pps S Cr203 NiO BaO CO 2 C LOl

Hp

2.63 0.80 5.00

J

3

4

58.93 18.17

62.15 11.38

39.05 29.07 0.51

48.06 19.31 1.56

9.55

6.09 24.97 0.34 2.22 1.36 0.95 0.03 0.36 0.06 nd

26.05 2.10 4.12 nd nd Tr 0.12 nd nd

J 2.90 0.13 0.40 5.78 0.92 0.03 0.12 0.005 0.02 0.004 0.05

5.06 4.01 nd 4.07 0.68 0.06 0.09 nd 0.01 0.001 0.04,

3.18

7.52

6

5

*

*

64.57 1l.93

J

13.47 1.31 0.87 0.84 0.80 0.02 0.20 2.27

4.12

1. Average shale (Pettijohn 1975, Table 8.7, averaged by Potter ef aI. 1980). 2. & 3. Precambrian shales from Varikunta-Zangamrajupalle, Cuddapah, India (Geological Survey of India 1986, source: V. Subrahmanyam). Naturally fused Permian shale ('para lava'), Bokaro Gondwana Basin, India (Fermor 1918). 4. 5. Naturally fused Permian shale (' para lava '), Jharia Gondwana Basin, India (Sengupta 1957). * The volatiles have escaped during spontaneous combustion and fusion. 6. Bijaigarh Shale, Precambrian, Vindhyan 'System' (Awasthi 1964, cited by Singh 1980). nd = not determined.

Composition and Classification oj Lutaceous Rocks

Besides clay minerals, which constitute the bulk, some grains of quartz, feldspar, carbonates and organic matter are found in all shales. The non-terrigenous constituents include glauconite, volcanic glass, biogenic silica and some phosphatic compounds. Shales may be classified by their colours, which reflect chemical composition. With increasing Fe 3 + IFe 2+ ratio, the colour of a shale changes from grey to red, through green and yellow (McBride 1974). Red shales, found near the surface, indicate the presence of hydrated ferric oxide minerals, which are formed under oxidizing conditions. Since an arid or semi-arid condition helps oxidation, red shales may be taken to be indicative of an arid palaeo-environment. A red colour is also common in deep-marine clays. This is partly due to oxidation of the ferruginous minerals carried into the sea by streams and partly due to the volcanic dusts that settle into the deep sea from the atmosphere. Clay-size ferruginous matter, settling through the oxygenrich sea-water at an extremely slow rate, may become oxidized over the years.

32


Similarity in the chemical composition of some deep-sea clays and an average igneous rock suggests their volcanic origin (Dunbar and Rodgers 1957, p. 57). The proportion of organic carbon is another important constituent influencing the colour of shales. With an increasing amount of organic carbon, grey shales tend to be black. The presence of pyrite and siderite together with trace elements such as vanadium, nickel and uranium in black shales suggests that these shales were deposited under a reducing environment (see discussions under euxinic basins).

Clay Minerals in Shales Clay minerals (hydrous aluminosilicates) such as kaolinite, illite and smectite are important constituents of shales. Gibbsite (aluminium hydroxide) is also common. The type of clay mineral present in a particular shale depends on the nature of the source rock, depositional environment, climate and also, to some extent, the degree of diagenesis undergone. Kaolinite is generally the product of warm, humid, tropical conditions of weathering. In areas of low rainfall the proportion of smectite increases compared to kaolinite. Illite, very common in shales, may be derived either from pre-existing shales in the source area or by diagenesis of clay minerals such as smectite. Chlorite can also be produced in this way. In fact, illite and chlorite are common in older shales while kaolinite and smectite are characteristic of younger shales. Depositional Environment of Shales A certain amount of low-energy condition is essential for deposition of all fine-grained sediments. The deposits of the deep-sea include blue-grey mud of terrigenous origin, green mud of glauconitic origin, and red mud produced either out of volcanic dusts or from clay-size particles of ferruginous sediments washed into the sea. Planktonic fauna may be associated with these deposits when they are laid down at relatively shallow depths. At greater depths siliceous fauna predominates. Ocean-floor clay deposits may extend for hundreds of kilometres. Elongate bodies of silt may be deposited on the inner and outer shelf areas, if the energy condition there is sufficiently low. Deposition of silt and clay is in no way restricted to a marine environment only. Finely suspended silt and clay spilling over river channels during high flood may produce extensive sheets of mud in the flood basins. Settling by flocculation on overbank areas also causes deposition of mud. In large lakes the coarser particles are trapped near the shore but the finer mud travels into the lake centre, to be dropped there from suspension. These form extensive, ovate muddy deposits at the lake bottom. Invertebrate shells of various types, spores and algae are common in these deposits. In closed basins, whether marine or non-marine, restricted circulation may lead to the deposition of black shales. Nearshore areas of fluctuating marine and continental conditions can also trap a large amount of mud. Deltas, lagoons and tidal flats are examples of mixed environments which can have large depQsits of silt and clay. Many shale sections in stratigraphic record demonstrate cyclicity. These may be either varves or non-varves, such as alteration of shale and limestone. Some of these features are discussed later (Chapter 7).


33

ROCKS OF CHEMICAL AND BIOCHEMICAL ORIGIN <;:arbonates

Sedimentary rocks of chemical and biochemical origin are divided into two groups: evaporites and non-evaporites. Non-evaporites constitute the bulk of chemical and biochemical rocks. Among non-evaporites the most important are the carbonate rocks. The carbonate minerals forming these rocks include calcite (rhombohedral CaC0 3), aragonite (orthorhombic CaC0 3 ) and dolomite (rhombohedral CaC0 3 .MgC0 3). Rhombohedral iron carbonate, FeC0 3 (siderite) also constitutes a small proportion of the naturally occurring carbonates. The term limestone is generally restricted to the calcitic and aragonitic rocks. Limestones composed essentially of the mineral dolomite are called dolostones. Approximately one quarter of the stratigraphic record is constituted of limestones and dolostones. Limestones occur in all ages, from the Precambrian to the Recent, but they are most common in the Late Precambrian (Proterozoic). Limestones are also abundant in the early Palaeozoic.

Chemistry of Carbonate Precipitation Pn;cipitation of calcium carbonate in nature is dependent on a cycle of natural processes termed the carbonate cycle. These processes involve formation of carbonic acid from water and carbon dioxide (C0 2 + H 20 <=> H 2C0 3) followed by dissolution of carbonic acid in two stages (McLane 1995, p. 228). H ZC0 3 <=> H+ + HC0 3RCO) ¢:> R+ + CO}Precipitation of calcium carbonate takes place according to the following reaction: ~ CaC0 3 Ca2 + The natural processes aiding in the above mentioned chemical reactions are: increase of temperature, evaporation, upwelling of deep marine waters to zones of lower pressure and higher temperature, photosynthesis, and bacterial decay. An limestones contain some quantities of Si, AI, and Fe. These are the acid insoluble residues (Table 3.7). In natural calcites the amount of MgC0 3 molecule is less than about 4 mole percent. These are the low-Mg calcites. High-Mg calcites contain much higher proportion (10-18 mole per cent) of MgC0 3• The chemical composition of some Indian limestones of different geological ages is compared to the composition of the well-known limestones from Europe and USA in Tabfe 4.7. An average carbonate rock, like the one tabulated in the first column of this table, consists of some quantity of MgO together with large proportions of CaO and CO 2, In fact, calcite (CaC0 3) and magnesite (MgC0 3) form an isomorphous series within which two varieties of calcite occur. One of the varieties designated as high magnesian calcite, contains .more than 10 mole per cent of MgC0 3 . The other, designated as low-magnesian calcite, contains less than 8 mole per cent MgC0 3 . This division is arbitrary. A calcite containing 9 mole per cent MgC0 3 may be designated either high-or low-magnesian ::~lcite (Friedman and Sanders, 1978). Carbonate sediments, it must be remembered, form close to the area of deposition either by inorganic precipitation or by organic activity. The chemical conditions. prevailing at the site of deposition therefore, play important roles in carbonate sedimentation. Oxygen isotopes in limestones have been used extensively for

cot


34

interpreting palaeotemperature and palaeosalinity. The problem in palaeotemperature interpretation is that the oxygen isotope is affected, not only by temperature variation but also by physiology and life habitat of organisms. Interpretation of palaeosalinity from oxygen isotopes gives more dependable and consistent results. Isotope studies can also be used in the interpretation of diagenetic and cementation history of limestones. Table 3.7: Chemical composition of limestones and dolomites (percentage of major oxides)

1

2

5.19 0.81

1.15 0.45

0.54 FeO MgO CaO Na20

KzO TiO z MnO

PzOs S

7.90 42.61

3 0.09

} 0.26

0.11

55.37

0.05 }

6

8.774 0.762

15.88 6.79

10.51 3.28

8.69

3.36

1.08 34.06 nd

15.49 25.79 nd

1.43

0.19

1.74 0.14 0.62 0.02 nd nd 0.03

LOI41.624 29.86

38.94

0.720 }

15.363 30.319 0.994

7

8

2.85 1.15

4.12 1.24 0.92

0.78 17.50 24.24 1.26

0.16 2.03 50.42 0.84

0.86 0.33 0.39

0.45

0.67 0.22 D.3D 0.36

0.55

D.77

38.26

38.65

11.44

0.07 0.04

0.33 0.06 0.05 0.04

Cr203 BaO

HP+

5

0.745 0.35

0.56 53.80

4

0.56

0.69

HP-

0.21

0.23

41.58

42.69

0.26 1.71

0.03 0.005 0.01

} 0.32

CO 2

0.784 0.141 C.1C5 0.109

43.11

1. Composite analyses of 345 limestones (Clarke 1924)* 2. Lithographic limestone, Solenhofen, Bavaria (Clarke 1924)* 3. Travertine Hotsprings, Yellowstone, Wyoming (Clarke 1924)* 4. Average composition of 38 limestone samples, Krol Formation, Simla Himalayas (Bagati 1979). 5. Limestone, 6. Dolomite, both from Varikunta-Zangamrajupalle, Cuddapah, India (Geological Survey of India 1986, source: V. Subrahmanyam). 7. Fawn dolomitic limestone (microdolsparite), Son Valley, lndia (Mehrotra et al. 1975). 8. Rhotas limestone (micrite), Son Valley, India (Mehrotra et al. 1975). Note: nd = not determined * Cited by Pettijohn 1975, Table 10.4.

Trace Elements in Limestones A number of trace elements, particularly, Fe2 +, Mn2+, Sr2+, constituting 10- 2 to 10-3 weight per cent of the rock, are also found in limestones (see TabJe 3.8). Ions of these trace elements repJace the host ions having similar charge and ionic radius. Later they find their way into the interstitial space between crystal boundaries. These trace elements attach themselves to the silicates, oxides or carbonates, or may OCCUr as substitutes of Ca2+-. Intensive studies on these trace elements have been carried out with the hope of

35


obtaining clues to the microenvironment of deposition and diagenesis of carbonates. For example, the proportion of iron and manganese in carbonates is determined largely by the state of oxidation (redox potential) during carbonate precipitation which in turn is controlled by organic respiration, bacterial decay, and photosynthesis. However, such environmental interpretations may not always be correct. Sr, commonly believed to be characteristic of lagoons and central reefs, is also found in evaporites and inland lake deposits (Hugel 1982). Sr in aragonite, once thought to be phylogenetically controlled, is now believed to be dependent also on environmental temperature. Nonreefal carbonate sediments are characterized by trace elements like Ni, Co, V, Ba, and occasionally, also Sr. The presence of sodium in carbonates is considered by some to be indicative of precipitation in saline water. High concentration of P in the carbonates from the Krol Formation of the Simla Himalayas has been taken to be indicative of near shore lagoonal environment (Table 3.8). Similarly, high concentration of Mn and P are believed to be indicative of warm, humid climate (Bagati 1979). (See Tucker et al. 1990, McLane 1995 for further details on trace elements in limestones). Table 3.8: Average concentration of trace elements (in ppm) in relation to carbonate microfacies, Krol Formation, Simla Himalaya (Bagati 1979)

SiliceIntrado- Mit-Tite Sandy Total no. oolite- losparuof micrite bearing samples dite oodolomicrudite 4 samples 4 samples 2 samples 5 samples 1 samples 4 samples 2 samples 1 samples 23

ChertDolomiTrace Dolomicrite bering crite elemedolomicrite nts #1 #2

Pb

Ga V

Bi Sn Cu Ni Cr Ba Sr Zn

60 68 59 27 30 69 50 19 91 112

50 145 187 60 44 45 35 200 160 25

25 75 30 30 25 15 135 13

Dolosparite

50 43 84 92 37 43 50 26 126 25

OilS

50 55 140 IS

45 50 75 140

25

70 113 14 129 57 42

50 21 99 131 112

58

97 162 28 19 50 82 175 25

50 88 62 20 50 50 25 90 290 40

- Sought but not detected. Note: For definition of rock names such as dolomicrite see text under dolomite.

Limestone Forming Environments Limestone is found today in almost all environments, both marine and non-marine (Fig. 3.9). Seawater being mildly alkaline (pH - 8) allows precipitation of limestone. Carbonate saturation in water is highest in tropical to warm, temperate seas. A small increase in temperature or alkalinity in these areas may cause the carbonate to precipitate. The type of carbonate sediments deposited on marine shelves is discussed in detail in Chapter 7, under facies associations.


36 Carbonate soil

Mean

~-::::l:::P_~

sea _ _---,_ _ Level

Fla. 3.9: Depositlo~al environment of non-marine carbonates. (based on Wolf 1978).

Limestones of non-madne origin include algal mounds formed in fresh water lakes. These are mixtures of carbonates and argillaceous sediments. The corresponding rock is called marlstone. Travertines in hot springs, spongy, .porous carbonate deposits (tufa) associated with natural springs; dense, elongate, vertical bodies (stdlactites and stalagmites) formed within caves, are other examples of freshwater limestone deposits (Fig. 3.9). All these are mostly calcitic limestones. In the arid,desert environment, and also in Some supratidal areas,dolomite occurs together with gypsum and anhydrite. The environment of deposition of this association is discussed under sabkhas (p. 154 footnote). In dry. and semi-arid regions thin layers of carbonate occur on the surface soil Jogether with some silica, alumina and iron oxide. These are produced by evaporation of Hme-rich water; drawn to ·the surface from the vadose zone by capillary action. These deposits, recognizable in ancient stratigraphic rocks also, are named caliche. Such features have been described from many localities including the Permian CaPitan reef complex of southern New Mexico and westTexas.(Dunh~m 1969). De indurated counterpart of caliche is the calcrete, also called kankar in India, nari mthe Middle East, ana kalkkruste lIT uermany. in Chile and Peru the term caliche is used for deposits of impure nitrate of soda together with sulphates, iodates and phosphates. Karst (named alter tne K.arst re$!lon~Of yugoslavia) imolies a particular type of topography well knowntor,lts scemc beautv. Carbonate cll~~olutl0n and nredpitation within caverns by percolatmg grouna water, anaun<1ergrouna streams are responsible for generation of karsts. Pisolites COnsIst or ,pea:Slzed .(2-1 U mm In diameter)· grains commonly made of calcium carbonate. Pisolites of other minerals like bauxite. limonite. hematite are also known. In cross~section plsolites Showconcentnc lam mae SImIlar to those found in oolites. Such fe~tures, 'often:fc)Uhdin; caves and mines, are also, called cave pearls. They are common in geyserartd hot spang depOSits; Generation of growth-laminae by chemical precipitation followed by their abrasion due to contact with neighbouring grains in agitated water is the process commonly suggested for thedeve!opmenf of pisolites (Donahue 1969).Somepisolitesofthe Carlsbad facies, previously regarded as algal onkoids produced in shallowlagoonbehindthe reef rock, have been shown


37

to be of diagenetic origin (Dunham 1969). However, 'pisoids' containing radial fibrouslayers in the Miocene reef talus of Egypt are of algal onkoid origin (Aissaoui et al. 1986). Spars are equant, blocky, anhedral chemically precipitated carbonates of fairly large size (a few micrometres to several millimetres). They are composed of low-magnesium calcite or ordered dolomite, but never of high-magnesium calcite. Calcite and dolomite sparr cement occurs very early in the diagenetic history of a carbonate rock. Some calcite spar (pseudospar) can also be the product of neomorphism of micrite or of fibrous carbonate cement (McLane 1995 p. 239).

JAfrbonate Petrography and Classification Limestones can form by precipitation within a basin or may be redeposited elsewhere, after erosion and transportation from their place of origin by water or air. The in situ limestones have been termed autochthonous by Pettijohn (Folk prefers the term orthochemical for such rocks) while the term allochthonous is used for the transported . variety (allochemical of Folk). Their texture and structure can distinguish the two types of limestones. While the grains of allochthonous limestones can be well sorted as to size and have their interstices filled with clear calcite cement, the grains of autochthonous limestones are usually unsorted. The intergranular spaces of such limestones are filled with lime mud. The allochthonous limestones show evidence of transportation, such as cross-bedded structures. The autochthonous limestones have articulated organic structures like undisturbed coral colonies. Carbonate rocks made up of materials eroded from older, lithified' carbonate sediments have been termed calclithitebyFolk(1959) todistinguisli them from calcarenites, a term coined by Grabau (1904) for rocks composed of carbonate sand produced by chemical or biochemical precipitation . . The transported and redeposited allochemical components of carbonates !tre divided into the following classes by Folk (1959). Intraclasrs; KewurKeu rragmems or penecomemporaneous caroonale semmenIS. Fossils (skeletalS): Broken or unbroken skeletons and tests of carbonate secreting organisms.

Oolites (ooids): Spherical carbonate bodies, 0.1 to 1.0 mm in diameter having concentric or radial internal structure. Pellets (peloids): Extremely small (0.03-0.15 mm) carbonate bodies lacking internal structures. Intraclasts are made UP of microcrystalline or fine-grained carbonates. Rippedapart fragments denved trom the underlymg deSIccated beds by strong currents otten constit.ute the intraclasts. These oft~n show'curl-up' desiccation structures. The fossils may range, from invertebrates like' foraminifer, corals, echinoderms, bryozoans; and brachiopods to broken skeletal remains of vertebrates or even shark teeth. The fossil assemblages found in limestones often characterize a particular geological age or environment. Oolites generally occur in clusters. Recent oolites are composed of concentric layers of aragonite around a nucleus, which mayor may not be a carbonate. Ancient oolites are generally calcitic. Oolites having radial internal structures often

38


alternate with those having tangential structures. Opinion varies as to whether the radial or tangential structures are of primary origin. Most of the pellets represent excretra of animals ('fecal pellets') but pellets of non-organic, chemical origin are also known. Tucker et al. (1990) used the term peloid to describe sand size, spherical grains composed of microcrystalline carbonate. These have been described as a group of polygenetic grains originating out of any of the following constituents: faecal pellets; calcareous algae, micritised grains, or simply mud clasts. The orthochemical components are divided into two groups: micrite and sparite.

Micrite: Microcrystalline calcitic deposits of a size comparable to terrigenous clay (0.001 to 0.004 plm). Tli'e constituent grains can hardly be resolved, except under high magnification. White or grey (with waxy to dull lustre) to nearly black in hang specimens, micrite is believed to be deposited as biochemical ooze or produced by attrition of organic shells. Some of the ancient micrites are possibly of diagenetic origin. Micrites may consolidate to form carbonate rocks.

Spllrite (tspar' or sparry calcite): Calcite crystals deposited within veins, pore spaces or fissures are called sparite. Individual spar crystals generally range from 0.01 to 0.10 mm in size. They may exceed 1.0 mm in diameter in exceptional cases. Some of the ancient spars are believed to be the products of recrystallization of micrite. Sparry calcite is deposited only as a secondary vein or pore-filling material. Folk's scheme of classification of limestones uses various combinations of th~ allochemical and orthochemicaI components (see Fig. 3.10). Each term used in Folk's scheme is divided into two parts: a preftx and a stem (Table 3.9). The preftx indicates the nature of the allochemical component (intraclast, fossil, oolite or pellet). The stem indicates the nature of the orthochemical component (micrite or sparite). The specialitY of Folk's scheme is that the nomenclature gives a complete petrographic description of the limestone. Thus Folk's scheme of limestone classification is so popular among petrographers. A quick method for on-the-spot classification of limestones was developed by Dunham (1962) with a view to helping well-site and field geologists. This classificatIon scheme is based on easily identiftable characters-whether the limestone is grainsupported or mud-supported. This scheme has been popular, particularly among petroleum geologists, because it provides a quick method of IdentIfication of rock properties useful for reservoir evaluation. A comparative study of the schemes proposed by Folk (1959), Dunham (1962) and Pettijohn (1975) is given in Table 3.10. The major groups of limestones identified by Dunham (1962) are described below (see Fig. 3.14). Grainstones of Dunham (1962) are rocks having a grain-supported framework The intergranular space is filled with clear calcite cement. The carbonate detritus may consist of fragments of pre-existing limestones (intrasparite of Folk), fossil fragments (biosparite of Folk) or oolites (oosparite of Folk). Depending on the grain·size of the constituting carbonate grams, a grainstone may be grouped either as calcarenite (sand-size grains) or calcirndite (larger than sand-size) of Grabau (1904).

39


The sandbanks of the Bahamas are similar to the ancient calcarenites. This has led many researchers to presume that the ancient calcarenites were deposited either in shallow seas under the influence of strong tidal action, or as subaerial carbonate dunes. Calcarenites, in fact, can be produced in marine conditions when oolites and skeletal debris of sand size are bound together by calcareous mud. Sparry calcite may also serve as the cementing material for calcarenites (Pettijohn 1975, p. 353). ORTHOCHEMICAL ROCKS

A)'.-LOCHEMICAL ROCKS Sparry Calcite Cement

III Microcrystalline Calcite Lacking Allocchems

II Microcrystalline Calcite Matrix

..... [J .

z

. ".

Intraclasts

::

o

.,

Intrasparite

E-< ell

0 p..

Oolites

:::E 0

:::E

•

Fossils

p-.l

~

Biosparite

U

.:.~, ..... :::.:"

~

:'.:@<@:.. . '.,

':"'~"':":" . '.

,

::~.:>.

.'

Dismicrite

"

Autochthonous Reef Rocks IV

Biomicrite

o

~ ''''' '' ':

..J ..J

«

0'

"

Oomicrite ' ..

.. .

Micrite

.. ~' ... C/~ .~ Oosparite

U

Intramicrite

.

.. ..:: ... :.:

. ':

Pellets

.

',

.

. . . .. ' . . ,'. .. . "

',

Pelsparite

BIolithite

Pelmicrite

~ Microcrystalline Calcite

o

Sparry Calcite

Fig. 3.10: Graphic-representation of Folk's scheme for classification of limestones (redrawn from Folk 1959), With permission of the author and American Association of Petroleum Geologists.

Boundstone is the same as reefoida1 carbonate. This represents a loose network of carbonate with large voids. The framework is constituted of organisms such as corals, algae and bryozoa. Wackestones are micrites having more than 10% coarse grains. These may be composed of skeletons of organisms or other materials.

Packstone has a grain-supported framework but contains a matrix of mud. Mudstone consists of extremely fine-grained carbonate, comparable to the calcilutite of Grabau (1904) or the micrite of Folk (1959). The fine·grained, perfectly homogeneous variety of mudstone used for lithographic purposes is called a lithographic stone.

40


Mudstones may be produced by attrition, or may be chemIcally or bIOChemIcally precipitated. The choice of the term mudstone to designate a particular type of limestone is not a happy one because argillaceous rocks of terrigenous origin and composed of silt-clay are also called mudstones (see Table 3.5).

Fig. 3.11: Photomicrograph of an oosparite from Vempalle Formation , Papaghani Group (Upper Proterozoic), Cuddapah, Vempalle, India (thin section No. A 11 /3, courtesy of O. Oasgupta and C. N. Rao) .

Fig . 3.12: Pnotomlcrograph ot a biosparite from 'Shell Limestone'. Trichinopoly (thin section No . 6, author's collection). Cross polarized light.

41


Fig. 3.13: Photomicrograph of an intrasparite. Locality unknown (thin section supplied by Hindustan Minerals & N.H .8 .S. Co., Kolkata.

Depositional texture recognizZlble Original components not bound together during deposition

i - - - - - - - - - - - - ---r--------i Original Conta ins mud Components Lacks mud (particles less than 20 mi crons) bound together t - - - - - - - - - r - - - - - - - L . . - - - - - i during deposition Grain-s upported

Mud-supported

grains

grams

mud

mud

<10%

>10%

>10%

<10%

Mudstone

Wackestone

Packstone

Grainstone

.

o· . ,- .

.Q . 6'0

.

.

. A •• '. '

'

..

. .

. . ().'C~ ' , . '0· ....

.' . . ·O:·J .' d ' .' 0: a ··~ • • 0

Texture not recogn izable

"

Boundstone

Crystalline Carbonate

.

'

Fig. 3.14:Dunham 's (1962) scheme of c lassification of carbonate rocks according to depositional texture (modified) .

Dolomite and Dolomitization When more than 50% of a carbonate rock is composed of the mineral dolomite (CaC03 .MgC0 3 ), the rock is called a dolostone (Shrock 1948) or simply a dolomite. Pettijohn would name a rock dolomite only if the proportion of dolomite in it exceeds


42

90%. When calcite occupies more that 10% in such a rock (between 10 and 50%), the resultant rock is named a 'calcitic dolomite.' A dolomite with an appreciable proportion of ferrous carbonate is called a ferrodolomite Cferrandolomite', Pettiijohn 1975 pp; 359-360). Ferrous carbonate entering into a solid solution with dolomite imparts a buff colour to the rock upon oxidation. In hand specimens dolomite is often identified by this colour, which is uncommon in calcite limestones. The other common impurities in natural dolomites are anhydrite and gypsum. Table 3.9: Folk's scheme of classification of limestones (1959). Terms proposed by Grabau (1904) and Pettijohn (1975) have been inserted for comparison

Dominant Allochemical Component Grain size

Orthochemical Component Allochems> 10% sparite > micrite

Intraclasts

> sand

sand

Oolites

> sand

sand

Fossils

INTRASPARRUDITE Calcirudite (Intraformational conglomerate)

INTRAMI CRUDITE

INTRASPARlTE calcarenite (lithic calcarenite)

INTRAMlCRITE

OOSPARRUDITE Calcirudite (pisolite)

OOMICRUDITE

OOSPARITE Calcarenite (oolitic calcarenite)

OOMICRITE

BIOSPARRUDITE Calcirudite (coquina)

BIOMICRUDITE (coquinoid limestones)

BIOSPARITE Calcarenite (biocalcareni te)

BIOMICRlTE (fossiliferous calcilutite)

PELSPARITE Calcilutite (Pellet calcarenite)

PELMICRITE (Pelletiferous calcilutite)

Microcrystalline rocks

(l)

...... M

u

E-" '>l

o ..... ....... 0 Q

'......

;:s

.......

'0 .: 1=:1 __ c
> sand

sand

Pellets

Allochems < 10%

micrite> sparite

u

.....

<::i

U '--'

...

u

~

Note: Terms proposed by Folk (1959) are given in block letters, those proposed by Grabau (1904) in italics. Those proposed by Pettijohn (1975) in parentheses.

In ancient geological records dolomites are more common than calcite limestones although, strangely enough, dolomite-forming environments are extremely rare in the world today. Only some of the sabkha flats of the Persian Gulf (see under evaporates), the playa lakes of California, the hypersaline lagoons of South Australia, the supratidal areas of Florida and the Bahamas are at present known to be congenial to the precipitation of dolomite.

43


Table 3.10: A comparative study of the major schemes of classification of limestones Crlferton

Alithonty

ClussljiwtlOn

Grabau

I

( 1904)

2 3

Mode of deposItIon

PettIjohn (1975)

1 2

Mode of depositIon

Folk (1959)

GraIn-SIZe

2

Textural features Nature ofbrndmg ofongmal components

> Sand sIze Sand sIze < Sand sIze

CalClrudlte CalcarenIte CalCilutite

PreCIpItated m situ Transported from outsIde

Autochthonous Allochthonous

PreCIpitated

Orthochemlcal Spante MIcnte Allochemlcal Jn traclasts Oolrtes FOSSIls Pellets

In

Situ

Transported from outsIde or locally deposIted

Dunham

Components bound together dunng deposltIon

(1962)

Name

Boundstone

Components not bound together 2 (A) Rock IS mudsupported grams < 10% Mudstone grams> 10% Wackestone 2.

2 (8) Rock IS gramsupported

Packstone Gramstone

Seawater is an abundant source of magnesIUm 1Ons, but dOlOmIte does not Widely precipitate in seawater. This is because presence of SO~- in seawater inhibits precipitation of dolomite. A little S01- is enough to inhibit dolomitization of calcite, but aragonite can still be dolomitized under this condition (Tucker et al. 1990, p. 369). Sulphate in solution, while inhibiting dolomitization, does not totally stop precipitat10n of dolomite_ In fact, in some of the coastal sabkhas dolomite and gypsum are known to precipitate together. Folk and Land (1975) studied the mfluence of salinity and Mg/Ca ratio on precipitation of calcite, dolomite and aragonite in different natural environments (Fig. 3.15). It emerged from their study that salinity and Mg/Ca content of the medium are the two most important factors controlling precipitation of calCite and dolomite. When salinity is low, dolomite can precipitate at a Mg/Ca ratio of 1: 1. With an increase in salimty, a much higher concentratIOn of Mg IS needed for precipitation of dolomite. Two types of dolomites are recogmzed: (z) primary and (iJ) secondary. The primary dolomites are very fine grained, smaller than 0.020 mm generally. These, named dolomicrite by Folk, contain unmistakable signs of primary deposition like laminations, ripple marks, and mud cracks. Microbeddmgs of calcite and dolomite, calcite intraclasts within dolostones, and dolostones laterally passing into clacitic limestones also indicate primary nature of dolomites. The secondary dolomites are

IntrodJctlon to Sedimentology

44

the coar~Lly cry\t
I 30

10

Mg

Ca 13

1I

3 1

)

10 1

10r------r-----.------.-~--_r~r_,_.__

30'1

350%0

35%0 Reflux DolomIte

Z

01

35%0

o

001

035%0 (350 ppm)

o RIVERS Q

0001 ~~~--~----~1~1-----3~1-----IO~I------~35ppm Mg/Ca Fig. 3.15: Influence of salInity and Mg/Ca ratIo on preCIpitation of calcite, dolomIte and aragonIte In different natural environments (compiled from diagrams by Folk and Land 1975)

Seawater entenng restncted lagoons evaporates fast, leavwg a hot, dense, alkalme and salme flUId (FIg 3 16) Thls water loses a part of Its calcIUm to the carbonate secretmg organisms Further loss of calctum due to precIpitation of calCIum bearIng evaporates like gypsum and anhydnte on landward shoals leads to hot, hypersalme bnne of hlgh Mg/Ca content Because of Its hlgh denSIty thlS bnne Invades the surroundIng

45


carbonates, replacing caldte by dolomite. This sfI:page ref7ux h)pOlhesis of Adams and Rhodes was very popular in the 1960s. However, Hsii and Siegenthaler (1969) doubted the efficiency of the seepage mechanism alone to dolomitize large masses of calcite of low permeability In their model of evaporative pumping they suggested that evaporation of the pore water in sediments draw phreatic seawater into the surface enriching its dissolved ion content and aiding in dolomitization. Fast evaporation

water Lagoon

Fig. 3.16: Schematic representation of the seepage-reflux mechanism leading to dolomitization (based on Adams and Rhodes 1960).

Localised replacement of calcite by dolomite along post-depositional structures like faults and fissures might have produced what has been termed as epigenetic dolostones. Syngenetic dolo stones could have developed penecontemporaneously by the reaction of lime carbonate with magnesium bearing seawater (Friedman and Sanders 1978, p.179). Observations on the coastal sabkha of the Persian Gulf led Patterson and Kinsman (1932) tD suggest that evaporation of seawater from the supratidal surface of coastal sabkha lead to precipitation of gypsum and accentuation of the Mg/Ca,ratio. This saline water of high magnesium concentration dolomitizes the surrounding calcareous sediment as it seeps into it. Mixing of the meteoric freshwater with seawater might result to a fluid of low salinity and high Mg/Ca ratio-a condition ideal for the deposition of magnesium. As the zone of mixing of freshwater and marine phreatic water moves across a body of carbonate, it dolomitizes the sediment (McLane 1995, pp. 246-247). Dolomitization involves large-scale recrystallization. The process often leads to the generation of a mosaic of euhedral dolomite crystals, producing a texture called idiotopic fabric. When the crystal faces are not so well developed a mosaic of anhedral dolomite crystal results (Fig. 3.1 This is called the xenotopicfabric (Friedman 1965). Intermediate textures are called hypidiotopic. As early as 1836 it was suggested that replacement of calcite limestone by dolomite results in a volume shrinkage of 12-13%. This idea, subsequently discarded, became popular again, following the studies by Holt in 1948. It is now believed that shrinkage of c-axis of crystals of dolomite creates voids during replacement. Because of the secondary porosity acquired in this way dolomite bodies may act as effective resen-oirs of petroleum (Levorsen 1958, p. 121). However. modern workers have pointed out that volume shrinkage can take place only when replacement occurs within a closed system. a situation rarely found in nature (Longman 19~ I)

n

46


Fig. 3.17: Sparry dolomite crystals filling a cavity in a limestone. Note progressive increase in crystal size towards the centre. Location: PUlivendla, India (thin section no. H 13(11), courtesy of O.K. Oasgupta and C.N. Rao).

Chemical Sediments Other Than Carbonates The chemical sediments other than carbonates belong to two groups of precipitates, primary and secondary. The primary precipitates include evaporites such as chlorides, nitrates and sulphates. The secondary precipitates include non-evaporites such as the . siliceous, phosphatic and ferruginous deposits.

Evaporites Evaporites are precipitated from solution and concentrated by evaporation. Sodium chloride (halite), and calcium sulphates (gypsum and anhydrite) are common examples of evaporates. Potassium chloride (sylvite), hydrated magnesium chlorides (bischovite and epsomite) are relatively rare varieties of evaporite. AlJ these rock-forming salts are deposited by the process of evaporation of sea-water. The sequence of deposition followed by the common rock-forming evaporites is as follows (beginning with the least soluble mineral, which is always the ftrst to be deposited): gypsum or anhydrite, sylvite and halite. Areas of extreme. aridity and local areas of internal drainage with a high rate of evaporation, as in continental or coastal sabkhas, are ideal for precipitation of evaporites. Another ideal situation for evaporite deposition is a barred (silled) basin formed as a bay or cut-off arm of a sea (see discussions on euxinic Or silled basins). Circulation in such a closed basin is maintained by the influx of salt-laden sea-water flowing above

47


the sill and return (reOux) of thIS watel Into the sea a'> d ~ubsurface current 1 he amount of evaponte deposited Into the closed baSIn IS governed by the amount of salt flowmg mto the baSin and the amount carned back to the sea dUrIng reflux In thIS process the least soluble salts (anhydnte and gypsum) arc depmIted near the SIll and the hIghly soluble salts (halIte and sylvIte) arc deposIted away from the SIll, In the dIstal part of the baSIn (see Chapter 7, FIg 7 20) Due to preCIpItatIon of the calcIUm IOns In the proxImal part of the basm m the form of gypsum and anhydnte, the sea-water becomes enrIched m magnesIUm. The magnesIUm-nch sea-water flOWIng back mto the sea causes dolomItIzatIon of the Canch carbonates precIpItated on the sIll ThIs IS what IS commonly called the 'reflux theory' of dolomltlzatlOn The physIcal envIronments of evaponte deposItIon and the mam facIes found In these enVIronments are summanzed In FIg 3.18. lncrea~mg ,..I

u

SULPHATE (;3

~od)

~

-<

(/)

.....1

-< r--

(/)

-<

c:ff)

SALT

@

Contmcntal

CIj

V)

.....1

-< ::c ~

)

I\.)

z 5 c bvo z"" ga(j ;:::l 1= .a u z ;:::l 0

Sahmty

,...

,.

'"' -.J

~ ...,

.

Sabkha

4

___

.

......... .--.. .--..

'"d ro ~ ~ ro.-C

-........,.,-,,-

1\.)0...

o~

lIZ

~

Coastal Sabkha

'"do ro ......

>~

,..s:::: VJ

0

U

Turbulcnce Increasmg

~~

~

C/]

::J 0

~

..9 c;j

..c

VJ

_

b

.

n

~

d

n

n

n

Do

n

a

LamlI1atcd

w.l

(lJ1Tf

0 CY

Crystallme wIth Carbonates

-< ~

0

~ CrossLammated

-----

~

RIppled

~~ '-"";/-"'"

Chevron· halIte beds

--==-----~~

~ ..:::::::::=-

Wavy, anastomoslllg beds

Lammae of hopper rafts HI

C/]

0.

0

~/rl

l:::Jo 0'0

Crystallme \\ nhout CarbO!1dtc,

Dcbn<;

\:},\"-/Q

nOlI.,

TurbidItes

Lammated cycle "alt~

Fig. 3.18: PhYSical environments of evaporite depOSition (based on Schreiber and Kendall 1984)

et at

1976

48


Evaporites are found in all geological ages, from the Precambrian to the Recent, ?ut they are possibly most common in the Permian. The best examples ar~ the Per~l1an salt deposits of Texas, Mexico and New Mexico. The well-known evaponte deposits of north-west Europe, Germany, and the North Sea are also of Permian age. Also well known are the salt deposits of the Persian Gulf belonging to a wide range of geological ages, from the Permian to the Tertiary. The Rann of Kutch of Western India, the coastal sabkha of the Persian Gulf, the Great Bitter Lak;e of Suez, the Death Valley of the USA and the Dead Sea are some of the well-known evaporite depositional areas of the present day.

Siliceous Sediments The common siliceous minerals of sedimentary origin include chert, flint, opal, chalcedony and a variety of other microcrystalline silica. Under the microscope they show the aggregate polarization effect of silica although the individual crystal boundaries cannot be recognized. They occur either in association with the shallowwater orthoquartzite-carbonates, or with the deep-water geosynclinal sediments. High temperature and a high degree of alkalinity (pH > 8.5) are known to increase the solubility of silica in water. A decrease in temperature and alkalinity allows precipitation of silica, either near the surface or in deep water. The origin of siliceous sediments can be either inorganic or organic. Silica may precipitate from sodium silicate gels in alkaline lakes. Silica of deep-water geosynclinal association is ascribed to silica-secreting organisms such as the oozes of radiolaria and diatoms. Some of the bedded cherts are believed to be lithified ooze of organic origin. Some of the chert can also be or secondary origin-the product of diagenetic differentiation and replacement (see discussions under diagenesis).

Phosphatic Sediments All sediments containing more than 7.8% PzOs (equivalent to an apatite content of about 20%) are classified as phosphatic sediments. Those containing more than 19.5% of P20S (apatite content of 50%) are called phosphorites. The phosphate in sediments is mostly of organic origin, derived from the hard parts of organisms, which extract phosphorus from sea-water. Guano (organic excreta) deposits are some of the best-known organic phosphate deposits found on land. Nodular phosphatic conglomerates and metasomatic phosphatic deposits are of inorganic origin. Like all other chemical sediments, the deposition of phosphatic sediments is controlled by the acidity-alkalinity and oxidation-reduction potential of the medium of deposition. A slightly reducing, alkaline condition provides the environment most congenial for deposition of phosphate (see Fig. 3.19). Phosphates can occur either as a platform facies, as in the glauconite-bearing marine sediments, or as a basinal (deepsea) facies produced under anaerobic conditions. In general, phosphate-bearing sediments are more common in the Phanerozoic than the Precambrian because of the abundance of organisms in the former. The phosp horites in India, however, are mostly of Precambrian or Cambrian age. These have been assigned to th~ee groups according to their association (Banerjee 1986): 1) bedded cryptocrystalline phosphorites associated with dolomitic stromatolites; 2) Intrac1astic


49

and microcrystalline phosphorites associated with carbonates, black shales and dark cherts; 3) phosphorites associated with ferruginous sediments and residual weathering products. All the above-mentioned phosphorites are of marine origin. Silled (restricted) basin conditions, controlled by basement topography, were responsible for the high concentration (10-35%) of P 20 S in most cases. The specific conditions of deposition of these phosphates have been discussed in detail by Banerjee (1986). In Rajasthan, a restricted condition was provided by the presence of a shallow carbonate embankment along a Precambrian basement ridge. Similar conditions also existed in the Himalayas where the phosphorites were deposited over the Upper Proterozoic Krol Formation. In'Peninsular India, dolomites of Proterozoic age provided the silled basin conditions for the Vindhyan phosphorites. Association with stromatolites in many cases indicates that micro-organisms played important roles in concentrating phosphorus from seawater. In some cases, as in the Bijawar rocks of Central India, processes such as diagenetic mobilization, weathering, leaching, precipitation in voids and fissures also helped in concentrating phosphorites. Phosphorites found in association with ironstones in the Raniganj Coalfield, India are of shallow-water origin.

Iron-bearing Sediments Iron-bearing sediments include banded (bedded) iron formations (BIF), ironstones, ferruginous ooids and fossil fragments. The bedded iron formations consist of alternating thin layers of silica and ferruginous minerals of chemical origin. The ironbearing minerals are mostly oxides (magnetite and hematite). Silica occurs as quartz, chert or jasper. The bedded iron rocks, found in all the Precambrian cratons of. the world, are often modified by diagenesis and metamorphism. The ironstone formations which are of younger age, are composed mostly of iron carbonate (siderito) with some iron silicates (chamosite). Ooids and fossil fragments, often replaced by iron oxide minerals, are also known to occur within ironstones. The siderites are often altered to limonite near the surface. As in the case of other chemical sediments, acidity-alkalinity (pH) and oxidationreduction potential (Eh) play important roles in the precipitation of iron-bearing minerals. As indicated by the Krumbein-Garrels diagram (Fig. 3.19), an oxidizing and alkaline condition is ideal for precipitation of the iron oxide minerals. The production of near-surface carbonate or bicarbonate of iron could be wholly inorganic (James 1966). The inorganic iron may be transported together with clay minerals as colloidal iron oxide. Organic matters can also play an important role in the transportation of iron because large proportions of ferric oxide can be kept in colloidal suspension by small quantities of organic matter. Following a more modern approach to the problem, Berner (1981) concludes that hematite and goethite are precipitated under oxic (Co 2 ~ IQ-6) conditions, while pyrite and marcasite are precipitated in anoxic (Co 2 < 10--6) conditions. Siderite is formed under anoxic conditions which are nonsulphidic (C H2s < 10-6 ). These conclusions are based on the measurement of dissolved oxygen and dissolved sulphide in modern sediments at the time of authigenic mineral formation. C in the notations refers to concentration of oxygen or total sulphide in terms of moles per litre.

50


The conditions leading to simultaneous deposition of silica and iron oxides are not fully understood but a marine or restricted marine environment of sedimentation, somewhat isolated from the open sea, is believed to be responsible for deposition of the banded iron formations of Precambrian age. A Precambrian ocean with an upper oxic layer and a bottom anoxic layer containing Fe and Si0 2 is the commonly assumed model. Large-scale precipitation of BIF followed the upwelling of deep, Fe-Si0 2-rich waters to the newly developed continental margins and shelves.

,,

+0.1

8.0

7.0

pH~

I

... ,

'--":'~~~

' .. ,p....... ... (;>

"

I

H~U.5.L Hematite Limonite MnOxides Silica Chamosite*

,

co ~

.. A~'

~~~

'0

i

II

] ~~ '-~~~~!.~~~~~--~~- MAT T E R FF....!;E:.....:.!N~C::..E::-~E....:h=---=_O;....L._ _

--""'O""'R""'-::O-:A-:N::-;-:'I c..-__---:

S

~ ~~ t.

!

Eh

St'll'ca

.....

Peat

~ ~

~

gtiS

~

'

~~

.

0 Ai

~ f'

Orgamc matter Siderite Glauconite Rhodochrosite Chamosite Phosphorite

SU L

Peat Pyrite

Organic matter Phosphorite Pyrite Silica

Calcite Organic matter etc.

.e~('

fM>H. 0 Salinity >200%0 Gypsum Anhydrite Halite

Dolomite

Organic matter etc.

Calcite Organic matter Cif'tb11 0 etc. SalInIty >200%0

Z

-0.3

4MI-( Salinity >200%0 Gypsum Anhydrite Halite Dolomite etc.

Calcite etc.

PlfID

E

Gypsum Anhydnte HalIte Orgamc matter etc FEN DolomIte

CE

o~ F-<

gj

~

>-l

Calcite Organic matter etc.

* Chamosite as used here is representative of the sedimentary iron silicates Fig. 3.19: Fence diagrar:n illus.trating influence of Eh and pH on generation of mineral assemblages In sedimentary rocks (after Krumbein and Garrels 1952).


51

In some cases, as in the Precambrian rocks of the Lake Superior District, silica and iron, produced from weathering of landmasses, were deposited in alternate layers under freshwater lacustrine conditions. The model proposed for such a deposition by Hough (1958) envIsages a lacustrine condition wherein settling of the dead, oxygen-consuming organisms from the surface creates a reducing condition in the lake bottom which is cold as well as acidic. The condition is ideal for precipitation of silica, while iron brought in suspension by streams during the summer remains in solution to be precipitated as iron oxide only when the lake 'turns over' (see Chapter 7) during the autumn to produce an oxidizing environment. Repetition of the process leads to the production of alternating bands of iron oxide and silica. Volcanic activity, sea-bottom reactions on clastic materials and deep-seated weathering processes on land are some of the commonly suggested sources of iron and silica. These conditions, it has been suggested, were repeated periodically but occurred on a unique scale in the early Proterozoic (2.0-2.5 b.y.), when an extraordinary concentration of ElF (92% of the total) was formed in almost all continents round the globe (Goodwin 1982). Erosion under climatic conditions similar to what prevails today will not allow transportation of large quantities of iron in suspension without a substantial amount of accompanying detritus, but surprisingly most of the thick sequences of the Precambrian iron deposits are devoid of clastics. As an explanation, it has been proposed that the Precambrian atmosphere had a large content of CO2, resulting in acidic surface waters, which allowed easy leaching and transportation of iron with no accompanying detritus. It is also possible that near-absence of oxygen in the Precambrian atmosphere did not allow oxides of iron, the chief inhibitor in the process of iron transportation, to form. Local concentrations of iron oxides in this model were explained as the result of local biological photosynthesis. As an alternative to these suggestions, it has been proposed that the sluggish surface run-off over a senile, deeply weathered Precambrian land surface did not allow transportation of particles coarser than particulate iron oxide together with fine clay. Many workers are of the opinion, however, that the contributions of volcanic, hydrothermal or biological processes in the generation of Precambrian iron deposits cannot be ruled out altogether (Stanton 1972). The Precambrian Banded Hematite Jasper (BHJ) Formation of Noamundi, India contains numerous syndepositional structures suggesting their deposition in a shallowwater environment. The possible biological origin for at least a part of the iron and silica is suggested by the presence of microstructures resembling organic forms (Rai, Sarkar and Paul 1980; Sarkar 1984). According to one view, the banded iron formations represent exhalative phases of volcanism along a fracture zone. Iron and silica, discharged into an inner sea, were precipitated out in alternating bands through this fracture (Banerji 1977). A source of pyroclastics and exhalatives (volcanic and/or hydrothermal) adjacent to the existing BHJ occurrence together with a rapid rate of precipitation of the detrital-pyroclastic/chemical sediments close to their place of generation has been suggested as an alternative hypothesis for the iron and manganese deposits of the Noamundi Basin (Dasgupta and Batabyal 1988). Ironstones are also known from the Ironstone Shale Formation of the Raniganj Coalfield in West Bengal, and its stratigraphic equivalent in other Gondwana basins of India. Apart from iron and silica, these rocks contain small quantities of phosphorus

52

Intr.>duction to Sedimentology

and manganese also (Walker 1914, cited by Krishnan, 1964). The assemblage of minerals in these ironstones suggest their precipitation in environments which were slightly alkaline as well as reducing. This and other lines of evidence have led to the suggestion of a marine invasion inro some of the Gondwana basins of India during the Permian (Bose and Sengupta 1993).

VOLCANICLASTIC ROCKS Sedimentary rocks constituted of fragments of volcanic origin are called volcaniclastic rocks. These rocks contain the products of weathering and erosion of pre-existing volcanic rocks or pyroclastic debris. Pyroclastic is the name given to rocks composed of the ejecta from volcanic vents. Volcaniclastic sediments are broadly divided into two groups according .to grainsize - the coarser volcanic breccia (angular fragments) and agglomerates (rounded fragments), and the finer tuffs. Breccia and agglomerates consist of volcanic bombs and rock fragments. Tuffs are composed of fine crystals, rock fragments and volcanic glass. These are named according to their principal constituent - crystal tuffs, lithic tuffs and vitric tuffs (Fig. 3.20).

Fig. 3.20: Photomicrograph of a tuff (plane polarized light). Locality near Padberg, Germany.

The origin of most volcaniclastic deposits is ascribed to eruption of rhyolitic lava which, being highly viscous, with a high gas content, is particularly explosive. The finer ash is carried to the upper atmosphere and transported for long distances. Deposition of ash may lead to extensive pyroclastic beds (' ash/alf). Clouds of hot gas ('gJowing cloud' or nuee ardente), produced out of some acidic voJcanic eruptions, may also be responsible for sedimentation of an unusual nature. The basal flows resulting from these eruptions, contain ash, lapilli and blocks which are kept in suspension by escapmg gases. The mass may flow downstream as a turbulent mixture. The deposits formed from this mixture are called ash-flow tuffs, welded tuffs, or ignimbrites. These


53

poorly sorted, poorly bedded units have compositions comparable to rhyolitic lavas. A single deposit of ignimbrite may be as much as l()O m thick. The ash-fall deposits, on the other hand, are well sorted according to fait velocity of the grains, with mean thickness and grain-size decreasing ~way from th~ source. Hence they can be used as palaeocurrent indicators (Scheidegger and Potter 1968). Silicified tuffs, called porcel/anites, reported from the Proterozoic Vindhyan sediments by early workers of the Geological Survey of India, contain fragments of quartz and feldspar of vitroclastic shapes. Wedge-shaped ftagments of feldspar indicate their pyroclastic origin CAuden 1933, p. 154). These VC0lcanigenic deposits contain bombs, lapillis and huge ash falls (Ghosh 1972). Lava flow~, tuffs and pyroclastic breccia around 'crater-like structures' have been reported from the Proterozoic sediments of the Godavari valley, India (Saha and Ghosh 1987). These tuffs show distinct colour lamination, parallely arranged lapilli, flat-ended and stretched pumice fragments (Fig. 3.21). These nave Deen asslgneu" Ib symettbmc' v01Camsm Ill' Froterozoic {s(ana arcs. Asfl-6eas, interbedded with late Quaternary sediments, ha're been reported from the Son and Narmada valleys of India . This ash contains angular glass shards and some accessory minerals. In the absence of any evidence of Quaternary igneous activity in the areas, this ash-bed has been tentatively assigned to neotect'onic activity along the Son-Narmada lineament, known to be an area of high heat flow (Basu, Biswas and Acharyya 1987).

Fig. 3.21: Photomicrograph of a deformed tuff-lava, Sommanpalli, Godavari valley, lndia. Length of the stretched pumice fragment is about 0.76 mm (courtesy of D. K. Saha).

DIAGENESIS The changes which sediments undergo between deposition and lithification under normal pressure-temperature conditions are calJed diagenesis. More recently, the

54


parameters of diagenesis have been extended to depth (4-7 km) and temperature (120 0 220 0 C) conditions where incipient metamorphism sets in. Near-surface (telegenetic), shallow depth (eogenetic) and deeper level (mesogenetic) changes are included in this definition (Burley, Kantorowicz and Waugh 1985).

Diagenesis of Siliceous Sediments Near-surface diagenetic changes are caused by the interaction of minerals with interstitial pore fluids. This may lead to dissolution of the existing minerals and production of new (authigenic) minerals. Authigenic growths (' overgrowths') of quartz and feldspar are common features at this stage. The newly formed quartz is added to the detrital quartz grains maintaining the same crystallographic orientation (Fig. 3_22). Authigenic precipitation of feldspar takes place at a higher order of pressure and temperature than that provided by near-surface conditions_ Excess potassium in the pore fluid is an essential condition for development of authigenic feldspar_ Dissolution features in feldspars and production of kaolinite as pore-filling material are common at this stage.

Fig. 3.22: Photomicrogrph showing authigenic overgrowth on a quartz grain in quartz arenite from the Upper Proterozoic Chandarpur Formation, Raipur, India (thin section courtesy of S .S. Das and C.N . Rao) .

At a shallow ~e~th, in a warm, humid, non-marine environment, bacterial activity may Impart a dlstmctly black colour to fine-grained sediments. Lateritisation takes place in warm humid climate whereas calcrete forms in warm arid to semi-arid


55

conditions. In a warm humid climate, when the pore fluids are alkaline and rich in calcium, sodium and silica, but deficient in carbonates, a variety of zeolites may be produced authigenically. When the climate is hot an dry ferric oxides are produced by oxidation of ferruginous sediments. These oxides impart a red colouration to sediments at shallow depths. In marine conditions a variety of clay minerals, illite, smectite, interbedded illite-smectite, may be authigenic ally produced. Quartz, feldspar and clay authigenesis is often followed by carbonate precipitation under marine conditions. At greater depth mechanical processes play important roles in diagenesis. Fracturing, crushing and squashing of detrital grains and wrapping of mica flakes around quartz grains are common mesogenetic changes. Reduction of porosity, partly due to compaction with increasing depth of burial but largely due to chemical processes, takes place. With increasing depth the porosity of sand changes only marginally while that of shale decreases logarithmically. Reduction of intergranular pore space is caused by increase of solubility at the contact of solid clastic grains under pressure. This phenomenon is called 'pressure solution' (stylolite-see Chapter 6 for details). Pressure solqtion may change the nature of grain contact from tangential to sutured through concavo-convex (see Fig. 4.13) with increasing depth of burial. A minimum depth of I to 1.5 km is necessary for pressure solution to be effective. Limestones and calcareous sandstones need lesser temperature and pressure for welding of grains than arenites. Finer grained rocks are affected more because they provide larger surface areas for grain contact. Thin mms of clay around quartz grains promote pressure solution, but larger quantities of clay deter the process by 'cushioning' (Tada and Siever 1989). Optical microscopes are of little use in distinguishing between overgrowths and pressure solution effects such as concavo-convex contacts. The cathodo-Iuminescence technique often helps in such cases because the nature of the luminescence differs for detrital quartz and overgrowths (Sippel 1968). With increasing (mesogenetic) depth of burial, clay minerals such as smectite and kaolinite are replaced by more stable products, illite and chlorite. With increasing depth of burial, in the Neogene sediments of the Bengal Basin, Bangladesh, a progressive decrease in the proportion of kaolinite and calcite and an increase in the proportion of illite by illitization of illite-smectite were noted. Within the same sediments the following order of diagenesis was reported: chlorite authigenesis, siderite crystallization, development of illite rims, quartz overgrowths and production of late Fe-calcite cement (Imam and Shaw 1984). Source of Silica Cement

On land silica may be taken into solution during weathering of rock-forming silicates (see Chapter 2). Kaolinitization of feldspar and diagenetic alteration of clay minerals may also release some silica. In the early stage of diagenesis the pores of quartz sand may be filled up by silica produced from weathering. Opal and chalcedony cements in a desert environment represent a transition between weathering and diagenesis. Solubility of both amorphous silica and quartz increases with an increase in temperature and~ alkalinity of the medium. A sharp increase in silica solubility is noted when pH exceeds 8.5. Hot springs and hydrothermal waters, which satisfy these

56


conditions, might serve as sources of silica in sediments. Precipitation of secondary silica by hydrothermal solution is a common feature. Solutions saturated with silica at high pressure-temperature conditions at depth become further concentrated when they arc pushed up to cooler zones by compaction. In deeply buried sandstones some silica may be produced by pressure solution. The silica thus produced migrates into clay-free, fine-grained layers where authigenic overgrowth can take place readily (von Engelhardt 1977). Silica released by clay minerals during diagenesis of shales might migrate into adjacent sandstones to be deposited in voids. In a marine environment the silica seems to be of primary origin, generated at the sea-bottom due to dissolution of diatom tests. This leads to concentration of silica in the pores of deep-sea clay and silt. Escape of silica saturated water from the pores of clay or silt to the intergranular space of sand might account for much of the silica saturation in sandstones of marine origin. In some cases, as in the Buntsandstein sandstones of Germany, silica cementation has actually been found to increase towards shale (Fuchtbauer 1974). Diagenesis of Carbonates Replacement of aragonite and high-magnesian calcite by calcite and consequent reduction of pore space are the two most important effects of diagenesis on carbonate sediments of marine origin. These processes are to be studied with reference to the physicochemical conditions prevailing at sea. In a coralline island for example, four different environments prevail. These are : (1) a marine phreatic zone, where the pore spaces are filled with saline water (il) a freshwater phreatic zone immediately below the exposed part of the island; (iiI) a mixed zone with brackish water in the pore space and (iv) a vadose zone occupying the space between the surface and the water table. The pore space here contains water as well as air (Fig. 3.23). In the marine phreatic zone, where salinity and Mg/Ca content of the sea-water are fairly high, aragonite is secreted by corals (Fig. 3.15). The porosity of freshly deposited aragonite may be as high as 55%, but micritisation and submarine cementation inside the grains may reduce this porosity to about 40%. Intergranular cement"!f,ion may further reduce the porosity to 5-10% only. The binding material here is prov~ed by coralline algae which secretes magnesian calcite. A lowering of sea-level following this primary stage of cementation translocates the carbonate to the freshwater phreatic zone. Decrease of salinity and of Mg/ Ca ratio in this zone causes sparry calcite to precipitate within the shells and tests, thereby reducing the porosity to about 10%. Interestingly, the original shapes of the shells are not altered during this conversion. This is due to the presence of a micritic envelope which allows the calcitic pore fluid to pass through it, without destroying the shape of the outer shell. The envelope itself is not involved in this reaction (Friedman and Sanders, 1978). In the case of cavity ftlling larger crystals are formed at the centre and the smaller ones remain confined to the inner rim of the cavity (see Fig. 3.17). In some cases cementation takes place round the outer boundary of the pre-existing grains in the form of syntaxial rims. The process has been termed aggrading neomorphism (Folk 1974).

ri ..c

N

tagnant

00/0

MICfwzanon & CementatIOn

Equant calcIte cement

Sand belt

Sea \.\atel JPohgonal RadIal fibwus bOullltan

e

\"

Dune

~

___
\~

\~

's.

Reef front

Sellwood 1986 with permisSion)

by

Deep water Crusts (Mg-caltItc)

OPEN SEA

roann --_~, Cemented oolite ./. S

Mallnc Plucanc EnVITOllmcnt

active

Hard ground formatIOn

PhreatiC Zone Stagnant

Freshwater PhreatIc Envuonmellt

,o/:>,...

---"'-2sl.U.!'-lE ___ --

,-_

""'C'

<:;

TepeeS

Mud belt

PLATFORM

Fig .3.23: DiagenesIs of carbonates In freshwater and marine environments (modlflred and redrawn after a compilation

Mel1lscus Water film

FreshwJter Vadose Envuonment

~~

~\Q3 'g ~
('\l

I

Beachrock

\vadose

!Marinel

8 .;:lu _____ ~~:~e~~-----ctlVe _- - - - -

LAND

CD

en

-.J

(JJ

'<

to

o

a

CD

-

'< "tJ

Di ....

:3

CD

3

a.

58

IntroductIOn to SedImentology

Neomorphlsm Involves recrystallIzatIOn of the fine carbonate mud mto coarser aggregate!'. ('mlcyospars'), 5 lO microns tn diameter Th1s happens dunng dIageneSIs

because fine crystals of carbonate are thermodynam1cally unstable MagnesIUm Ions released to the surface of mIcntes dUring dIageneSIS tend to inhIbIt crystal growth but they are often flushed out by meteonc or deep connate water WhICh are defiCient m magnesIUm content When the carbonate IS left ill the freshwater zone for a long time, leachmg of aragonIte and Mg-calclte takes place by meteOrIC water Since these mmerals are unstable under freshwater condItIOns Although POroSIty may Increase temporanly at thIS stage, neomorphIsm of calcIte WhIch follows, fllls up the moulds, thereby reducmg the porOSIty to about 5-10% A vuggy porOSIty may develop, however, If the lImestone IS brought up to the vadose zone and retained there for a suffiCIent length of tIme The secondary POroSity developed m thIS way may prOVIde space for holdmg hydrocarbons Much of the reserVOIr porOSIty of the ou beanng Bombay HIgh' structures located off the western coast of IndIa, for example, IS due to the vuggy poroSity developed 1ll the vadose zone (Roychoudhury and Deshpande 1982) DISsolutIOn of carbonate and feldspar In the post-cementation phase, leadmg to generatIon of secondary porOSity, has also been reported from the Neogene sedllnents of the Bengal Baslll, Bangladesh (Iman and Shaw 1984) Although vadose zone leachmg makes an Important contnbutIon to the development of pOroSity, prolonged exposure of a carbonate rock til the vadose zone may cause a reduction til poros1ty ThiS 1S effected by preCIpItation of sparry cabte w1thIn the vugs When the aragomte IS of manne ongm, bIcarbonate WIth a poslt1ve value of bC* IS produced because the mean bC of manne carbonate IS POSitIve On the other hand, the humus-denved bicarbonate occurrmg III the near-surface vadose zone has a hIghly negatIve bC The reaction between these two bicarbonates produces a pore flUId WIth OC rangmg between -6 and -10, values not as hIghly negat1ve as the vadose zone bicarbonate The conditIOn IS Ideally sU1ted for preCIpitation of sparry calCite In the vadose zone The process may drastIcally reduce the porOSIty The occurrence of both SIlIca and carbonate cement m the same sandstone has been a mystery PreCIpItatIOn of carbonate IS pOSSIble only when the medlUm IS alkalme, but solubIlIty of s!l1ca 1S known to Increase With an mcrease In alkahntty When an algal mat growmg over a carbonate sediment IS penod1cally exposed to atmosphere, the mterstlttal water attams exceptionally h1gh alkahmty, leadIng to supersaturatIOn, and eventually preCIpitation of sllIca on carbonate ThIS IS one of the speCial conditlOns wherein SImultaneous preCipItation of SIlIca and carbonate may take place (BaasBecktng, Kaplan and Moore 1960) In India, a case of replacement of early diagenetic rhombiC carbonates by s11lca was reported by Mukhopadhyay and Chanda (1972) from the Iron Ore Group of rocks of Onssa Another Interestmg post depOSItional sequence of replacement of calcIte by secondary sIhca III the Lameta Beds of India, followed by repreClpltatlOn of calcIte from percolatmg waters, was reported by Chanda (1963) *oC represents deVIatIOn measured In parts per thousand (per mIl or %0) from the standard The standard IS taken to be a belemmte from the Cretaceous Pedee FormatIon 10 North Carolma USA

59

SedImentary Petrology

APPENDIX Techniques of Sedimentary Petrography

Scheme for Description of Sandstones A. Framework Grains 1 Mmeralogy

und~r

the Microscope

(a) Non undulatory (b) Undulatory (c) Polycrystallme (2 - 3) crystals/gram) (d) Polycrystallme > 3) crystals/gram)

Quartz

Feldspars Rock Fragments - speCIfy MIcas - muscovIte/blOtlte Heavy Mmerals - speCIfy Other Mmerals - speCIfy The proportlOns of quartz, feldspars and rock fr'agments should add up to 100 Use FIg 3 24 for Visual estlmatlOn of proportIOns AI~o de term me proportIOns of quartz grams of dIfferent vanetIes separately, so that they add up to 100

0 1%

2%

15%

25%

-. G) . ........- , ~

5%

,

10% '

50%

Fig. 3.24: Charts for estimating percentage compOSition of minerals under the microscope Prepared by M S Shvetsov (based on Terry and Chilingar 1955) With permission of SEPM (Society for Sedimentary Geolc)gy)

2 Texture (z) Gram-Size

use pre-calIbrated eyepIece mlCrometer for SIze deter mmatlOn See Table 4 1 for Wentworth nomenclature of SIze classes De~Ignate proportIons also In qualItatIve terms abundant, frequent, few, rare, e g , medIUm sand SIze partIcle~ abundant


60

(ll) Sortmg

use the followll1g scheme of classIficatIOn

Sortlllg

MutrIX content

Well sorted

Low

Moderately ~orted Poorly sorted

Moderate (- 10%) HIgh

Gram-size variatIOn

Most grams he wlthm one or two Wentworth SIze classes Moderate Large

Use Powers' chart (FIg. 4.9) for visual estimatIOn. For computatIon of the quantItatIve value of roundness follow the procedure dIscussed In Chapter 4 SphenClty (see FIgS 4 8 & 4 9). (iv) Nature of gram contact-see Flg 413 for comparison

(Ill) Roundness &

B. CEMENT- IdentIfy type-sIhceous/calcareovs/ ferruginous/any other. C. MATRlX - determme proportIon (greater or less than 12 5%) Name the rock (wacke or arenIte) accordmgly

D. CLASSIFICATION AND NOMENCLATURE - use revIsed Dott's (1964) scheme (see FIg. 3.3), e g, ArkosIc Wacke E. COMMENTS ON THE POSSIBLE NATURE OF THE SOURCE ROCK Use proportIons and types of quartz, feldspar and rock fragments (see discussions on provenance m Chapter 2) Plot 'a', 'b', 'c' and 'd' types of quartz on Fig. 2.4 to draw conclusIOns regardIng Igneous or metamorphIC nature of source rock. Note An easy way to classIfy a sandstone IS to determme whether the total matrIX content IS greater or less than 12 S%, I e respectIvely. whether the matrIX fills one·half of one quadran t of the

field of the mIcroscope The rock may be named aremte or wacke dependmg on whether the matrIX content IS smaller or greater than 12 5% respectIvely ProportIOns are to be determIned 10 three stages (I) proportIOn of matnx (11) proportIOn of the three Important constltuentsquartz, feldspars and rock fragments (III) proportIOns of four vaneties of quartz undulatory, non-undulatory etc

Scheme for Description of Limestones under the Microscope A. ALLOCHEMICAL COMPONENTS I

Intraclasts 2 Fosstls 3 OoIttes 4 Pellets

Determme proportlOn and sIze (sand SIze or larger) of each of these and tabulate In Folk's scheme (Table 3.9).

B. ORTHOCHEMICAL COMPONENTS MICrIte (microcrystalhne ooze, 0001-0 004mm) Spante (sparry calc1tlc or dolomitIc mosaIc, 0.02 - 0 1 mm) DecIde name of the lImestone by using appropnate combmatlon of A and B Use Folk's scheme of classIficatIon (see Table 39) Add 'rudlte' as suffrx if the maJonty of the alJochemical grams are larger than sand SIze

4 SEDIMENTARY TEXTURE

The term texture refers to the pattern formed wlthm a sedImentary rock by the constItutmg grams ThIS can best be studIed under a mIcroscope In sedImentary rocks of mechamcal ongm the framework grams are Jomed together, eIther by fmer fragmental partIcles or by chemIcally preCIpitated cement The former IS called matrzx and the latter, cement The pattern formed by framework grams, matnx and cement IS called clastiC texture Sandstones show thIS kmd of texture The texture of a rock of chemIcal ongm IS produced wholly by chemIcally precIpItated materIals No rntergranular space IS to be found rn such a rock LImestones of orthochemIcal ongm show thIS kmd of texture Allochemlcal lImestones, produced by fragmental pIeces of calcareous matenal Jomed together elther by fine carbonate mud or by calclte spars, show a clastIc texture (see Chapter 3 for defimtlOn of terms) ClastIC textures are defined mamly by SIze, shape and roundness of the framework grams Other features whIch play Important roles III clastIC texture are the nature of gram contact (gram-to-gram relatIOnshlp) and gram packlng PorosIty and permeabilIty of a clastic rock depend on these parameters

GRAIN-SIZE Concept of Particle Size

The defimtion of particle SIze IS related, at least m part, to the techlllque used for measunng It When a particle IS large enough to allow dIrect measurement of ItS dIameters, say by calIpers or by a mlCrometer scale fitted to the mIcroscope oculars, SIze can be expressed m terms of the dIameters (long, short and rntermedlate) A problem anses when a gram IS too rrregular to allow dIrect measurement of Its dIameters. In such cases the SIze IS measured mdrrectly, frrstly by measurmg Its volume, by the process of dIsplacement of water, and then by workmg out the dIameter of the sphere of eqUivalent volume by the followmg formula V 4/3rrr3, where Vand rare the volume and radIUS of the spehere respectlvely The dIameter thus obtamed was named 'true nom mal diameter' by Wadell tn 1932

=

Udden-Wentworth Size Scale Fragmental partIcles constitutmg sedlmentary rocks cover a WIde range of gratn-Slzes61


62

from boulder, which by definition is greater than 25cm, to clay which is less than 0.004 mm in diameter (less than O.002mm in Friedman and Sanders 1978, classification). Since handling of such a wide range of grain-size is a difficult problem, a geometric scale is used instead of an arithmetic one. In the so-called grade scale proposed by U dden in 1914, the consecutive figures in the scale increase by multiples of 2 on the higher side and decrease by multiples of V2 on the lower side. The various size ranges within the grade scale were named by Wentworth (1922) as follows: Clay Boulder I Cobble I Pebble I Granule I Sand I Silt 11256 (mm) 64 4 2 1116 256 The Udden-Wentworth system not only provided an appropariate scale for representation of grain-size but also standardized the sedimentological nomenclature. Adoption of this system of nomenclature by a committee of geologists and hydrologists dispelled the confusion created by arbitrary assignment of names to different parts of the grade scale by various authors. The Udden-Wentworth grade scale and nomenclature system are universally used today.

Phi Scale In the early days of sedimentology, when mechanical or electronic computing facilities were not readily available, handling of fractional values of the Udden grade scale involved tedious computations. As a solution to this problem, Krumbein (1934) introduced a method of logarithmic transformation as follows: ~ =-log2 d, where d =diameter of the grain in millimetres. Conversion of phi (~) to grain diameter (d) is attained as follows:

d = 2-4> The use of 2 as the base of the logarithm allows direct conversion of U dden's grade scale to Krumbein's phi scale. Moreover, inversion of sign converts the fractions to positive, round integers, thereby making easier computations of the finer grains (sand, silt and clay), which constitute the bulk of sediments. Log scale condenses the large range of grain-sizes of sedimentary particles thereby helping graphical representation. These advantages popularized the phi scale throughout the world for nearly half ,a century. However, with the advent of modern electronic computing devices, handling of fractional values being no longer a problem, the phi scale is losing its popularity. Sedimentologists are returning back to the millimetre scale. The combined Udden-Wentworth-Krumbein grain-size scale is shown in 1'able 4.1. Grain-Size Masurement

The techniques commonly used for grain-size analysis are: (z) direct measurement (in macro- or microscale), (iz) sieving and (iii) settling. Grains larger than sand can be measured directly by means of calipers, while an optical device, varying from a hand lens to a microscope, is needed for measuring rmer particles. Sieving is generally used for particles ranging from granule to fine sand. Settling techniques are used for sizes finer than sand (silt and clay). More sophisticated methods such as image analysis of

63

Sedimentary Texture

SEM plctures (without llltermediary photographs), electncal reslstlve pulse and lrghtblockage counters, disc centnfuge and laser scattenng devices are avalldble now (see Tables 4 2 and 43) The pnnClples behllld the techmques tradltlOnally used for gramSIze measurement are bnefly dIscussed below Table 4.1: Grain-size classification and nomenclature

Boulder Wentworth nomenclature

I

Cobble

256

(2 6)

I

-8

I

Sand

(22)

I

I

-1

Clay

1/16

1I256mm

(2 1)

(2-4)

(2 8 )mm

I

I

I

I

I

I

I

-6

-2

-1

+4

+8

Sand I V Coarse

Silt

2

I

I

I

(28)

Granule

4

64

I

Udden's grade scale Krumbem's phi-scale

I

Pebble

I 00

Coarse

I

+10

MedIUm

I

+2

I

V finel

Fme

a

(¢)

+3

a

+4

(<(l )

Gram-slZe measurements conducted under a microscope are mfluenced by the shape as well as the onentatlon of the grams When the grams are onented at random, the 'maxImum hOrIzontal mtercepts' through the grams are measured The SIzes thus obtamed are obVIously smaller than the actual sizes because seldom does the lme of traverse pass through the gram centre Theoretical studIes as well as expenmental observatIons have shown that the data obtamed by thIS method are some 24% smaller than the actual m most cases Thus the average radlUs of a sphere, computed on a random sectIOn by the techmques of maXImum mtercept measurement, IS 0 736 of the actual radIUS of the spheres (Krumbem and PettIjohn 1938, p 130) To take care of the shape vanatIOn of Irregular grams, Wadell (1935) recommended computatIOn of the area of the gram by a plammeter on an enlarged pIcture The nom mal sectional diameter, whIch IS the dIameter of a CIrcle of the same area, can be obtaIned from thIS measurement For elongate grams the three dIameters - a, b, and c, must be measured separately and the mean dIameter then computed by one of the folJowmg formulae: anthmetlc mean, da = (a + b + e)/3 harmOnIC mean, d h = (3 abc)/(ab + be + ca) geometnc mean, d g =

VJX

ComputatIon of the anthmetic mean suffices III most cases On average, the harmOnIC mean lIes wlthm about 6% of the anthmetlc mean

Sieving Technique SIeves are screens of standard SIze In the commonly used SIeves speCIfied by the Amencan SOCIety for Testmg MatenaIs (A S T M ), the openmgs mcrease UnIformly m multIplIes of if2. ' startmg from an openIng of 00029 Inch (200 mesh) The millImetre eqUIvalents of these openmg~ he qUIte close to the Wentworth grade lImIts and can be used for sedImentologIcal purposes A hst of the standard ASTM SIeves together WIth theIr mm and phI eqUIvalent IS gIven In Table 46 III AppendIx


64

Table 4.2: Techniques commonly used for grain-size analysis (modified after McCave and Syvitskl1991)

Altered Atterberg Gravel

Modern method of analysIs

Modified Udden Wentworth

Classical method of analysIs

Gravel

Count

Sand (5 grades)

SIeve

Settle Count Scatter hght

Silt (5 grades)

Settle

Count Settle Scatter hght

2mm

Sand (3 grades) 63J..lm SIlt (3 grades) 2~m*

Settle

Settle Centnfuge -------molecular - - - - - - - - - - - - - - - - - - - - - - - - Clay

Clay

* ModIfied sut-clay boundary after Fnedman and Sanders 1978 Table 4.3: Techniques of grain-size analysis applicable to different size ranges (modified after McCave and Syvitski 1991)

Range Count

S,eve

Top

la Ib Ic

I I Ie

Xd

X

X If /g Ig

Gravel Sand SIlt Clay Symbols

Pnnctple Settlefrom SuspensIOn

X X

Ih Ih

Laser scatter

X It /J

Xl

Photon correlatIOn spectroscopy

,X X X

/J

/ = techmque applIcable; =techmque ofhmlted apphcabIltty or Impossible, =Measurement on outcrops and photographs, b = Measurement on thm sectIOn or gram mount either directly or by Image analyzer, c = Image analysIs of SEM photographs or by P?lse and hght blockage counters; d = ElectrIcal resIstive pulse method measures down to -0 5 J..lm; e = Commonly measures down to 44 11m, m speCIal cases to 20-25 Ilm or even down to 5 X a

l.un by speCial Fntsch sieve shaker, Settlmg tubes measure weIght, differential pressure on pan, Visual accumulatIOn or f lIght attenuatIOn method of sensmg, g DISC centrifuge method Sediment IS mtroduced via a denSIty gradient and sensed by lIght attenuation DlscnrnmatHm down to 0 \) 1 ~m IS claimed; h Homogeneous settlmg suspensIOn Measurement by Simple pipette method, sedimentatIOn balar.ce, withdrawal from bottom, lIght or X-ray attenuation technIques I :::: Laser forward scattenng deVices; :::: Coulter and many other slmllar counters

= = =

Sedimentary Texfure

65

In the sieving technique a known weight of sediment sample consisting of loose grains is placed on top of a stack of screens arranged from bottom to top in order of increasing sieve diameter. When the stack is shaken, the particles divide into several fractions, depending on the number of sieves in the stack. The fraction lying on each sieve is collected separately and weighed. The theory of sieving assumes that the weight of material lying on a particular screen is proportional to the weight of the original materiel I of that particular size in the sample. Moreover, the sieve diameter of a grain is assumed to be the same as its nominal diameter. The theory of sieving, based on these assumptions, has many drawbacks. An elongate grain can pass through a square or round hole screen vertically. The smallest diameter of the grain involved in this case, may not even be remotely related to the nominal diameter. Moreover, a grain having a lon~ diameter 1.4 times the sieve opening can pass through a square mesh sieve diagonally, t}1e diagonal of a square of unit length being ..J2 = 1.41. Common manufacturing defects 01 sieves alSO aCta to fne prdti.lem. Square mesh sieves often stretch into rhombic sl1apes after prolonged usage, thereby increasing the effective diameter. Moreover, in this method, the grains are sorted according to their shapes as well as sizes. Flakes of mica, for example, lie flat on screens far' bigger than their nominal diameter, while elongate grains, passing vertically, accumulate on the finer sieves. , Interpretation of the results of sieving is also not straight forward. The conventional techniques of statistical analysis demand measurement of frequency (number counts), whereas in sieving the total weights of grains lying on each sieve are measured instead of numbers. This weight is represented by a complex parameter, frequency (n) multiplied by the mass of each grain on the sieve (M), or W=n.M, Where M = (4/3 7tr') p, 4/37tr' being the volume of a spherical grain of radius rand density p. This leads to additional complications because density of the grains influences size measurements. Moreover, r, the sieve opening, is assumed to be the same as the nominal grain diameter. In view of these complications, parameter W, the total weight of grains on each sieve, is designated as 'weight frequency' to distinguish it from true frequency (n). Appropriate techniques for analysis of weight frequency data not being available, the conventional techniques for analysis of frequency data are used for computing statistical parameters of grain-sizes from sieve data. Some attempts have been made to formulate new techniques of computation of mean and variance of the weight frequency data (Adhikari, Roy and Sengupta 1981), but the metpods developed are too sophisticated to be discussed here.

Settling Techniques In this method the particle concerned is allowed to fall through a column of water and the size of the grain is worked out from its fall velocity (v), using Stokes' equation: 2(pI - P2)g r2 v = --'---'--:::-,--::':""::'---9 11

66


where PI and P2 are the densities of the gram and the flUId respectIvely, 11 = coefficIent of VISCOSIty, g = acceleratIon due to graVity and r = radlUs of the gram concerned. The Stokes' equation can therefore be simplIfied as follows v=C·y2, all parameters, except the grain dIameter (r) and fall veloCIty (v), bemg constant for a partIcular expenment. Hence the radius of the particle can be worked out from its setthng velocity. The type of eqUIpment commonly used for measuring gram settlmg velocity IS Illustrated in FIg. 4.1. The partIcles concerned are allowed to fall through a tall (about 2m) column of water and the weIght of the grains accumulatmg on the balance pan immersed in water is recorded automatically agamst tIme. The advan tage of thIS system is that each gram of the sample is allowed to fall through the whole column of water and hence the curve obtamed is a direct record of the cumulative weight percentage of each sIze fractIon. Vanous other models of settling tubes, measunng fall velOCItIes of fine particles from suspension densIties, are available today. Modern versions of settlmg tubes use pressure-sensItive devIces or hght attenuatIOn techniques for measurement Computenzed systems m modern settlIng tubes take care of such variables as water temperature, grain shape, grain densIty, providmg gram-sIze dlstnbution patterns of the samples directly. In spite of these developments the setthng method cannot be used for rneasunng grains of all sizes because of the precondItions necessary for Stokes' law to be valid. The more important of these are: (1) the particle concerned IS sphencal, (2) the fluid medIUm is homogeneous, (3) the medIUm IS of unlImIted extent, (4) a constant settlmg velOCIty is attamed dunng fall of the partIcle through the fluid column and (5) the settlmg veloCIty of the grain IS not too hIgh. The lower size limIt for which the above-mentIoned condItions are valid bes between 0.15 and 0.5 ~lm. On the coarser SIde, 60 /lm is believed to be the limit (some would extend It to 80 /Jm). Free fall of grams larger than this size IS hmdered by generatIon of wakes (low pressure zones), thereby invalidating Stokes' law. These limitations confine the applicabIlity of Stokes' law to the SIlt and clay sIze grains only. Equations were developed by Oseen in 1910 and Rubey In 1933; to extend the limit of settlmg analysis to coarse silt. The rates of settlmg of grains coarser than this sIze vary as the square root of grain dIameter (see Krumbein and Pettijohn 1938, p. 101). In vIew of the difficultIes mvolved in converting settling time to grain dIameter, theoretical or empmcal curves relatmg settlmg velocity to gram diameter are often used for conversion (see, for example, FIg. 4.2). A comparative study of the different techinques of grain-size measuement IS presented in Table 4.4. In spite of its drawback, the sieving method 1S still very popular for gramsize measurement because the equIpment IS relatively inexpensive and the results are replicable, that IS, overall reproduclbihty is good Some techniques for conversIon of sIeve ('weIght frequency') data to thin section (number frequency) data are avatlable (Friedman 1958, 1962; Sahu 1964). Fnedman (1962) empmcally estbhshed a hnear correlation between plots of thm·section quartIle parmeters and slevmg quartIle parameter (FIg. 4 3) This prov1des an easy converSIon of one to the other MathematIcal models for comparison of sIeve and thm·sectIOn data are also avatlable (Burger and Skala 1976).

67

Sedimentary Texture

threads Water level

100%

61 11

I

IMinute

Settling tube

I

204 cm

88p ]04)1

55%

Recorder

.---'---,

B

A Fig. 4.1: (A) Schematic representation of the Gronlngen sedimentation balance for gralnsize measurement by settling technique. (8) Sketch of a grain-size distribution chart obtaln,ed from the Gronlngen balance (modified after Sengupta and Veenstra 1968 With permission of Elsevier Science Publishers B. V.).

Analysis and Interpretation of Grain-Size Data Grouped data of grain-sizes, obtained by any of the methods described above, are represented in tabular form. The attnbutes of grain-size frequency distribution (central tendency, dispersion, symmetry and peakedness) are then computed, either graphically or by using the technique of moment measures Moments are preferred to graphiC plots because they take care of every part of the frequency dlstributlOn. Followmg the

68


procedure gIven in the AppendIX, the first four moments can be readily calculated usmg a desk calculator (see Tables 4 7-4 11 III AppendIx). USIng a sUItable software, the results can be obtained In seconds on an electroDlc computer It IS a good practice to plot the results also as histogram, frequency polygon and cumulatlVe frequency curve (Fig. 4.4) The visual impresslOn created by these plots often helps m arnvlOg at geologically meanmgful conclusions. Before the modern electroDlC computers came into use it was a common practice to compute the statistIcal parameters m terms of phi-units with the help of the percentile values read off the cumulative frequency curve. The 'mclusive graphic measures' proposed by Folk and Ward (1957) were very popular for computational purposes for many years (Table 4.8. in Appendix) The particles produced out of the natural processes of weathering and erosion of a source rock are further sorted dUrIng transportation and deposition. The mechanIsm of grain sorting, which involves selective eltmmatlon of certain grain-sizes, vanes from environment to environment Gram·size parameters of sedIment populatIons from various modern environments have been studied intensively for more than half a century with the hope of estabhshmg a direct correlation between the two. The techmques used in most cases mvolved plottmg of one of the parameters of gram-size dlstnbutlOn against another with the intention of dIscnmmating between different environments (Friedman 1961, 1967; Passega 1957,1964; Sahu 1964a; MOIola and Welser 1968). Fnedman tried combmations of vanous statistical parameters and found plots of thIrd moment ('mean cubed deviation') against standard deVIatIOn (square root of second moment) to be one of the most effiCient methods for envIronmental dlscnmmatIOn (Friedman 1979). The principle involved in this discrimination can be demonstrated by referring to Friedman's earlier publicatIOns (Fnedman 1961; also see Figs. 4.5 and 4.6 of thiS text). River sand shows poorer sorting than beach sand because of the lower energy conditions prevailing in rivers. RIver sands are also distmgUlshed by positive skewness because of the presence of fines. The finer particles are washed away in the beach, leading to negative skewness. A perfect discriminatIOn between beach and nver sands can thus be obtamed by plottmg standard deviation (sorting) against skewness (Fig 4.5) In figure. 4.6 the (eohan) dune sand IS finer than the beach sand, and IS also positively skewed because the fines are retained in it Beach sand IS coarser because the competence of the ocean wave is greater than that of the wmd. Plottmg of mean size agamst skewness therefore, discriminates beach from eolian dune sand. In another line of approach popular among sedlmentologIsts, the different modes of sediment transportation are Identified by plottmg the cumulative frequency distributions of gram·sizes onprobabilzty paper (Fig. 4.7). A perfectly symmetncal ogive, which is the cumulative plot for a normal dIstributIOn, plots as a straight Illle on logprobability paper. When several such straight Imes show up on the log-probabilIty plot, as in figure 4.7. It is assumed that the sedIment population has been transported by dIfferent mechanisms, each of which has led to a normal dIstnbutlon The net distributIon is then diVided lOto traction load (coarsest fractiOn), suspensIOn load (finest fractIOn) and saltation load (medIUm fraction). Usmg thIS technIque, Visher (1969) IdentIfied a variety of transportatlonal·depositlonal conditiOns on the assumption that specific combmatlOns of traction, suspension and saltation load charactenze particular enVIronments

0.02

0.06

0.2 0.4

2 0.8 1 4 Velocity (em/sec)

6 8 10

20

60

0.00 100

0.02

0.04

c5

<':l

E

Q)

~

'-

g

E

/""'.

Fig. 4.2: Terminal fall velocities of quartz spheres of different diameters In water at different temperatures, as determmed experimentally by the U.S. Department of Agnculture, SOil ConservatIOn Service (modified and redrawn after Vanonl, Brooks and Kennedy 1961).

0.01

'- - - - - . 20 0 e ~ - - - - - - -. 30°C

I

, '----lOoC

0.2 0.08 0.06

0.2

0.4

1.0 0.8 0.6

2

4

6

8

-

en

<0

m

e-

)(

;nt

'<

....

I II

::J

CD

en CD a. 3

70


Table 4.4: Advantages and disadvantages of various methods used for grain-size measurement Technrque of measuremmt

Advantages

Dzsadvantages

Direct method cahpers or microscope (ocular micrometer)

ObservatIOns recorded as frequency (number counts) Hence statistIcal computations are easy Shape and roundness can also be measured together wIth gram-sIZe

TedIOUS and time-consumIng Measurements obtamed are less (by about 24%) than actual value ConversIOn to weight frequency IS reqUired for companson With sieve data

Sieving method

Easy to use EqUIpment mexpen slve Overall reproducIbIlIty of results IS good ThIS techmque IS widely used Hence companson wIth results obtamed else where IS possible

PartIcle shape and denSity mflu ence measurements Square hole sieves create speCIal problems Manufactunng defects are common In sIeves Procedure, though not as tedIOUS as mIcroscope measurements, IS tIme-consumIng Measures 'weIght frequency', a parameter for whIch the statistIcal mehod of analYSIS IS not yet fully developed

Settling method

RapId Easy to use ContInuous record of settlmg tIme (velocity) avatlable automatIcally by modern eqUIpment Shape, volume specific gravIty of grams are taken mto account m settlmg VelOCIty TransportatIOn and deposItIOn m nature are controlled by gram settlmg veloCIty, hence thIs IS the most dependable parameter for hydrodynamIc InterpretatIon

Shape and denSIty of grams mfluence settlmg veloClty but not exactly In the same way as In slevmg Hence companson WIth results of slevmg IS not easy ConverSIOn of settlmg VelOCIty to gram-sIze has to be done by formulae whIch have lImIted apphcabIltty, or WIth the help of emplncally denved curves based on observatIOns on quartz spheres Hence mterpretatlons are of lImIted value Settltng tubes of extraordmary length are requued for handlmg the whole range of sedIment sIze Natural samples tend to fall as one umt thereby artIfiCIally mcreasmg settlmg velocIty

The one-to-one correlatIOn between segments of log-probabIlIty plots and specIfic modes of sedIment transportatIon assumed III thIS type of InterpretatIOn, has raIsed doubts III some mInds (Komar 1986). SedIment populatIOns transported el ther wholly as bed load or wholly as suspended load III expenmental channels have been found to be segmented III log-probabIlrty plots (Sengupta 1975, 1979), suggestmg that these are not necessanly the result of transportatIOn by more than one mechalllsm The degree of log-normalrty attamed by a sedIment population IS perhaps a measure of

71

Sedimentary Texture

the flow parameters rather than depositIOnal environment In another study segmented log-probability plots, commonly mterpreted as mixtures of several lognormal polulatlOns, have been found to fit log-hyperbolIc better than log normal dlstnbutlOns (Chnstlansen, Blceslld and Dalsgaard 1984) The recent findmg that log-normalIty may develop out of a log-hyperbolIc dlstnbutlOn when the sortmg process IS nearly completed by a hIgh veloCity current, has proVided new clues to the mterpretatlOn of gram-size data (Sengupta, Ghosh and Mazumder 1991, Sengupta, Das, Majl, 1999)

40 35 00

!=:

~ .....

o~

30 ~

S ..c 25 E--<

0

20

~

'-' Q.)

~

(/)

15

..S:l u .....

~

~

r" o

.--.. 'J) ..... .....

10

/

/

/

V

/

V

'/

o

V ,

..

(/)

8

V

V'

..

,0

,

'

0',

,

0

,

05 ".

o o

05

10

I5

20

25

30

35

40

45

PartIcle SIze ( U mts) Thm SectIon Fig. 4.3: Single overall regression line for determining Sieve-Size distribution from thin section analysIs (after Friedman 1958, with permission of the author and The Unvlerslty of Chicago Press)

ROUNDNESS AND SHAPE Roundness and shape of clastic particles are two separate parameters, mdependent of each other. Roundness Implres roundrng of the edges and corners of a clastiC gnan Shape IS expressed as the ratio between the three measurable dIameters of an Irregular gram: length, breadth, and width Other measures of shape, mcludrng sphenclty and flatness, are also used


72

100

--------------------------------

I

~---'-)

I

~ Cumulative Curve (Ogive) I

---------------------------

80 I

Percentiles

,

P5=-O.825~

I,,

P 15 == - 0.225 4> P 25 == - 0.125 t\> (Q3) P so == 0.812 ¢ (Q2) P 75 == 1.325 ¢ (QI) P S4 == 1.525 ¢ P95 == 2.000 ¢

60 ~

~ ~

C q
J:......

40

"51

~

,

20

I

I

Hist ogram \ Frequency \ Polygon

-1.0 -0.5

0.0

0.5

1.5 Gram Size 1.0

2.0

2.5

3.0

3.5

4.0

(~)

Fig. 4.4: Graphic representation oj grain-size \requency data. Histogram, frequency polygon and cumulative curve for the data shown in Table 4.7 in Appendix.

Roundness

Following WadeU, roundness is defined as the ratio of the average radius of curvature of the edges and corners of a grain to the curvature of the maximum inscribed sphere. Measurements on spheres being difficult, two-dimensional projections of grains are generally used. Roundness in such a case is expressed as: Roundess

= (~ ~ / N) / R

where rl 5 are the radii of the corners of the particles, N = the total number of corners measured, and R is the radius of the largest possible inscribed circle (Fig. 4.8).

73

Sedimentary Texture

+250

+150

, II>.

I

.',. ,

"I

••

+050

'"

Vl

~ -0 50 ~

I

s -.. ,

,." •

.... ~,

B E A C' H. ,

(/) I

S A N.. D

-1 50

E

·R

A''N D' • River Sand • Beach Sand (Ocean) A Beach Sand (Lake) • Beach Sand (Ocean)

..,.\. \.

.. I:..

'"'"

V

R

I

.\ ,I \ I I I I I \

-250

\ ,

\

I

-350 Standard DevlatlOn (SOltmg)

Fig. 4.5: Plots of sorting vs. skewness of grain-size dlstnbutlon for discrimination of river and beach sands (from Fnedman 1961) With permission of the author and SEPM (Society for Sedimentary Geology)

120

100

<:..J N

080

Vi

Dune Sand Beach Sand (Ocean) • Beach Sand (Lake)

S

"" ~

C)

060

~

"" Cl)

::E 040

020

ooo~~--~~--~~--~~--~~--~~--~~~

-1.0

l 0

30

50 70 Skewness

90

110

Fig. 4.6: Plots of skewness vs mean grain-size for discriminatIOn of beach and (eolian) dune sand (from Friedman 1961) With permission of the author and SEPM (Society for Sedimentary Geology)

Introduction to Sedill'lentology

74

-CJ -CJ

;,;, -CJ

-CJ

'";,;,

-CJ

'"

00

'" -CJ

'"

00

'" 0 '" V>

~

:!2. e.....,

C r::

00

;::l

-.J

0

Il)

0

0-

....

a-

Il)

0

J.L;

V>

E

0

.~

*" '-" 0

Il)

;S

0

0

.~ ~

N 0

"5

0,)

(j'J

~

:g

~

0....

p..

......

E u

0

;::l

lJ>

0 N

V, ""

'" N ;.... '" '" <=> Vo

8'"

-·4.0

Grain-Size

(~)

Fig. 4.7: Probability plot of grain-size distributiof) of sand Sample No. USR 23-A from the Usri River, India (data shown in Table 4.7 in Appendix).

According to Wadell's definition, the roundness of a sphere is 1.0_ The nearer a natural particle is to this value, the more round~d it is. A chart giving photographs of pebbles of different roundness values was published by Powers (1953) to provide a quick aid to roundness determination by visual comparison (Fig. 4.9). Verbal expressions of roundness as given by various authors are shown in Table 4.5. Table 4.5: Comparison of roundness values of Pettijohn (1975), and Powers (1953).

Roundness tenns Very angular Angular Sub angular Subrounded Rounded Well rounded

Pettijohn (975)

class limits

midpoints

Powers (/953) midpoints

0.00-0.15 0.15-0.25 0.25-0.40 0.40-0.60 0.60·1.00

0.125 0.200 0.315 0.500 0.800

0.145 0.210 0.300 0.420 0.595 0.850

75

SedImentary Texture

Fig.4.8: Diagram Illustrating technique of measurement of grain roundness on an enlarged projection Roundnss

017

012

I

High SphenClty

Low SphencIty

= (~~t

0145

I

N} R :::: (81 1/16) mm/34 5 mm

o 35

025 0210

() 0

0~

Very Angular

/

Angular

I

0300

I

0 49 0 420

~ ~

~ Subangular

I

1 00

0 70 0 595 .

I

0 850

I

~ I)

, ~

Sub rounded

= 015

Rounded

~

Well rounded

Fig. 4.9 : Chart for estimation of roundness of sand grams of high and low sphericity by vIsual comparison (redrawn from Powers 1953) With permission of SEPM (Society for Sedimentary Geology)

Roundmg of sand-s1ze part1cles of quartz and feldspar 1S neglig1ble durmg fluv1al transport Whatever roundmg 1S attamed dunng transportatlOn 1S confined only to the first few kilometres and that, too, observable only m the pebble-s1ze grams (F1g 4 10) Beach actlOn 1S an effectlVe agent for roundmg of pebbles but not for sand Roundmg of a sand gram 1S belIeved, on the bas1s of expenmental stud1es, to be essent1ally due to eohan actlOn. The effect of solutlOn m the roundmg of quartz grams has been shown to be neglIgIble (Kuenen 1960 a, band 1964) For grams larger than sand, sIze and roundness generally show a pOSit1ve correlat1on Th1s 1S so because the smaller grams, wh1ch are produced by ch1Ppmg of the corners of


76

a large pebble, are more angular than the remaimng central part. The fact that pebble roundness is controlled by size of the pebble rather than distance of transportation puts a serious limitatIon to the use of pebble roundness as a palaeocurrent indicator. Size and roundness relation in sand sizes is more complicated. For example, Banerjee (1964) showed a negative correlation between SIze and roundness of sand and explained this as being the result of mixing of first-cycle and second-cycle sand grains

040 030 020~~~--&---·

010

©

020

Mean Roundness of Sand

010 I

I

I

I

I

I

I

I

I

I

I

I

I

I

,

,

,

,

,',

f

190 150

Longltudmal Profile of the Dwarkeswar River

110

70

o Dt,tance from Source (km) )

Fig. 4.10: Downstream change in roundness of pebbles and sand in the Dwarkeswar River, India (after A K. Mukhopadhyay 1988. unpublished M.Sc theSIS. liT, Kharagpur) .

Shape Shapes of sedimentary partIcles can be defined m many ways. A simple shape classificatIon, mtroduced more than half a century ago (Zmgg 1935), used the raito of breadth to length (bl a) and thickness to breadth (el b) of the particle concerned Four different shape nomenclatures (tabular, equant, bladed and prolate) were proposed on the baSIS of these ratios (Fig 4 11).

77

Sedimentary Texture

10

\ 6 ~7

\ 5

41 I

, \1 X \

\

\

\

8

1'--1<: 3 1 Tabular or oblate - .,- -,-, -- -, --, " , " \

2/3 6

~ ~/

\ \

K:' ~

, \ \

"" ,

", ,

... ....

'-.....

2

-...

....

~

---

I ....1..... I

I I -'I

- ----

I I I I I I I I I

V

I I I

I 2

6

4

, ~ ~')

~

1'''-/ , , ..... "- , ,,

"tx

... ....

---

-

, II ......Equant

--~ r-~~-

"-

III Bladed

o o

'-.1

"-

....

"-

I I

, .....

", , ...

9',

'or-. ... ....

I "-

~, I)

,

,

" , t',

- -

~-

I

I I 'i \ I

"/ ,

\

\

4

~ V·l~

1\

\

',8,

213

....

- .... -

-- I'r'"---- - ---

"-

3

....

~

----

I-

1- - - _

v '" IV PrOlate 8

10

clb Fig. 4.11: Curves of equal sphenclty supenmposed on Zingg's (1935) scheme of classification of pebble shapes Note that pebble 1 (oblate) and pebble 3 (prolate) fall In the same sphenclty class (compiled from Pettijohn 1975, Figs 318 and 3 19)

In another approach to shape measurement the closeness of a gram to the shape of a sphere (sphenClty) IS determmed by the ratlO sl S, where 's' IS the surface area of a sphere of the same volume as the partIcle havmg a surface area of'S' For an Ideal sphere thIS ratIo IS 1 For practical purposes, followmg Wadell, the dIameter of a sphere (dn) havmg the same volume as the particle m questIOn IS compared to the dIameter (Ds) of the smallest sphere circumscnbmg the parucle of Irregular shape The Wadell sphencity mdex m that case IS represented as d/ Ds Smce measurements of spheres are dIfficult, the dIameter of a partIcle of Irregular shape and that of the cucumscnbmg sphere are compared wIth the help of two-dImensIonal prOjectIOns It IS dIfficult to establIsh a duect relatIOnshIp between WadelI's sphencity mdex and Zmgg's shape for a sedImentary partIcle Pebbles of entIrely dIfferent Zmgg shapes (oblate and prolate) may have the same or SImIlar sphenCIty A more modern measure of eqUidimensionahty ot gram~ was mtroduced by Sneed and Folk (1958) for analYSIS of forms of partIcles The tnangular dIagram (FIg. 4.12) used for thIS purpose has three end members compact or sphencal (all axes equal), platy or oblate spheroId (two long and one short axes) and elongate or prolate spheroId (one long and two short axes) A gIven partIcle, WIth all three axes unequ aI, can occupy any field wlthm thIS trIangle, dependmg on (L - I)/(L - S) ratIO, where L, I and S represent the long, mtermedlate and short dIameters respectively The forrn cI,rses


78

obtained by this method give a better quantitative estimate of the pebble shape than the Zingg method.

0)0 Compact 1.0

rLl

s ~/? ~~~.5 a ~

.4

/"'7J 0 t::::::JI'

Q

o

Platy

.1

.2

.3

.4

.5

L-I L-S

.6

.7

.8

.9

.3 .2

..I.::""'L T

1.0 Elongate

c/

go Bladed

Fig. 4.12: Graphic method of determination of sphericity after Sneed and Folk (1958). Procedure: compute S/L and (L·I)I (L-S). The sphericity of the point at the intersection of these two parameters is obtained by interpolating between the curved lines. L, I, and S represent the long, intermediate and short diameters of the grain respectively. (With permission of The University of Chicago Press). Shapes of the grains of the end members are after Fritz and Moore 1988.

A new shape property, 'raJlability', was proposed by Winkelmolen (1971). Rollability is measured by the time taken by a grain, put into a slightly inclined, revolving drum, to roll down to the bottom of the drum. The average rollability value of a given sample is calculated per sieve fraction. The shape distribution character (SDC) obtained by plotting the relative rollability values against the size fractions, is claimed by Winkelmolen (1972) to have genetic significance. Shelf, shore and alluvial channel deposits of the Lower Oligocene of Belgium were shown to have distinctly different SDC characteristics. The shape of a mineral or rock fragment, often controlled by its cleavage or fracture, is rarely modified by prolonged trClnsportation. Analysis of particle form led Sneed and Folk (1958) to conclude that sphericity depends mainly on the inherent abrasional properties of different rock types and is a function of size as well as distance of transportation. It is little affected by selective sorting. In the course of another study,

Sedimentary Texture

79

Dobkms and Folk (1970) concluded that roundness of Isotroplcally weanng basalt pebbles mcreases from flvers to beaches, while sphenCIty decreases and the oblateprolate mdex becomes progressively negative, Ie, dIsc-lIke The productIOn of dISCS, whIch abound on beaches, IS the result of abraSIOn caused by pebbles slIdmg back and forth over sand or smaller pebbles m the surf zone SelectIve shape sortmg IS also Important, at least m some cases, as sandy beaches are capable of trappIng the dlscs GRAIN-TO-GRAIN RELATIONSHIP

Packing Arrangement of the framework grams wIthm a clastlc sedImentary rock and also the nature of contact between the adjacent grams, control what IS commonly termed 'packIng'. Packmg of the grams In turn controls porOSIty and also, IndIrectly, permeabzllty Point contact, long contact, concavo-convex contact and sutured contact are some of the terms commonly used m descnbmg the nature of gram contacts (FIg. 4 13)

.a·· ·.·0·.·.·. . .'· . .. ..

0

'

Floatmg Grams

.

. .. . :.~ ....... : .... . ..... . -':::;.-:: .. . . . . '. . . . ... .

. . .' .

. ····0· .. '.. .. .

.

.'

.

.

'

..

.

Pomt Contact

Long Contact

ConcavoConvex Contact

.~.

~.

Sutured GraIns

Fig. 4.13: Common types of grain contacts found PettiJohn, Potter, and Siever 1987)

In

clastiC sedimentary rocks (based on


80

When the matnx proportIon IS very hIgh, grams sImply' float' m the matnx Concavoconvex contact generally ImplIes solutIOn effect, whereas solutIOn, together wIth pressure, produces sutured gram contacts In most other cases, when the rock IS unaffected by solutIon or pressure, pomt, tangentIal or long contacts are found The mdex of packmg of the framework grams may be determmed by countmg the number of gram to gram contacts m a randomly cut sectIOn of the rock The result may be expressed eIther as 'packmgproxlmlty' or as 'packmg densIty' The former IS defined as the ratio of number of gram to gram contacts to the total number of grams counted m the traverse The la tter IS the ratIo of the sum of all gram mtercepts to the total length of the traverse ThIS, as PettIjohn (1975, P 75) has pomted out, WIll have an mverse relatIOnshIp to porosIty In thIS method of computatIOn It IS assumed that there IS no matnx or cement present m the rock A comparISon by PettIjohn shows that packmg (number of gram contacts) IS hIgher m orthoquartzites than subgreywackes Loose sand and calcareous oohtes have a much smaller packmg mdex than most sandstones

Porosity PorosIty or the proportIOn of VOIds withm a rock IS a functIon of all the textural parameters, gram-sIze, shape, sortmg, packmg, as well as the amount of v01d fillmg matenals, matnx and cement The absolute porosIty of a rock IS gIven by the ratIO - total pore volume/bulk volume of the rock For practical purposes, for example to assess the usefulness of a rock as a reserVOIr for petroleum, the e!foctlveporoslty IS more Important than the absolute POroSIty ThIS IS gIVen by the ratIo of the mterconnected pores to the total bulk volume of the rock The effectIve POroSIty IS generally 5 to 10% less than the absolute por~lty It IS the effectIve porOSIty that determmes the space occupIed by extractable flUIds However, not all flUIds occupymg pores can be extracted because m very fine pores the lIquId adheres to the pores due to surface tensIon POroSIty IS determmed to some extent by the manner of packmg of the gr¥TIs wIthm a rock An arrangement of packmg whIch Increases stabilIty of the grams Invanably decreases POroSIty For thIS reason a stable rhombohedral arrangement of the grams wIthm a rock produces a porOSIty of only 26%, whereas an unstable CUbIC arrangement produces a POroSIty of 47 6% (FIg 4 14) PorOSItIes of reservOIr rocks for petroleum are graded as follows 510% (poor), 10 15% (farr), 15-20% (good), 20-25% (very good)(Levorsen 1958, p 98)

,

~ ,~

/~

..

90° ,.

_____

,

~

r __

I

\

..

~

A

Fig 414 Influence of grain packing on porosity (A) cubiC packIng (porosIty = 48%) (8) rhombohedral packing (porosIty = 26%) (after Graton and Fraser 1935) With permisSion of The University of Chicago Press

Sedimentary Texture

81

PorOSIty can be determmed under the mIcroscope In thm se('tlons of rock specImens Cores or well cuttmgs collected from boreholes are mvarIably studIed under a bmocular mIcroscope for eStImatIon of POroSIty The concerned rock IS then qualitatIvely clasSified as tight, dense, porous or cavernous Porosity may also be estImated from such parameters as speed of dnllmg A sudden decrease m dnllIng speed may mdIcate an mcrease m POroSIty of the rock formatIOn because loosely packed, porous matenals can only be dnlled slowly In Situ determmatlOn of porOSIty can be done m the subsurface usmg vanous geophysIcal logs Tht'se methods have been dIscussed m the section on wmhne loggmg m Chapters 9 and 10

Permeability PermeabIlIty IS the property allowmg passage of flUId through a rock The umt of permeabIlIty IS a darcy Accordmg to the Amencan Petroleum InStitute, a porous medIUm IS saId to have a permeabIlIty of one darcy 'when a smgle phase flUId of one centIpoIse VISCOSity that completely fills the VOIds of the medIUm WIll flow through It under condItions of VISCOUS flow at a rate of one centimeter per second per square centImeter of cross sectIOnal area under a pressure of eqUIvalent hydraultc gradIent of one atmosphere (76 0 cm of Hg) per centImeter' The rate of flow of a flUId through a porous medIUm IS dependent on hydraulIc gradIent All rocks become permeable to some extent when the pressure dIfference IS suffiCIently hIgh or the flUId VISCOSIty IS suffiCIently low Temperature therefore exerts an mfluence on permeabIlIty HIgher temperature lowers flUId VISCOSIty thereby malung an otherwise non-permeable rock permeable Gram shape and packmg are the two other Important factors controllmg permeablhty of a formatIOn A decrease 1ll gramSIze lowers permeabIlIty by Increasmg capIllary pressure Moreover, permeabIlIty In many cases IS an amsotropIc property controlled by features such as beddmg planes, gram onentatlon and fractures In the laboratory permeabIlIty IS det~rmmed by forcmg a lIqUId through a core sample of the rock concerned by means of a pump and measunng the pressure drop across the sampLe on a manometer The eqUIpment used IS called a permeameter In Situ measures of permeabIlIty can be obtamed m boreholes by means of loggmg techmques U ncemented gravel and sand WIth a permeabIlIty of 10-10 5 darey are good aqUifers Those With lower darey values are poor aqUIfers Clays WIth darcy values of less than 105 are ImperVIOUS In the petroleum mdustry the umt of permeabIlity used IS a millldarcy (md =0 001 darey) Rocks are classIfied as follows according to therr permeability farr (1010 md), good (10-100 md), very good (100-1000 md) (Levorsen 1958)


82

APPENDIX Techniques of Granulometric Analysis Determination of Grain-Size Frequency ('Weight Frequency') Distribution by Sieving 1 Gram-sIze frequency ('weight frequency') dIstnbutlOns of loose sand SIze grams can be determmed by the followmg method Well-cemented sandstones must be dlsaggregated (wIthout breakIng the framework graIns) before SIeVIng Sandstones with carbonate cement can be easIly dIS aggregated by bOllmg ChIPS of the rock In dIlute Hel Ferrugmous cement m sandstones can be dIssolved by bOllmg ChIpS of the rock In dilute Hel together wIth granules of tm (Sn) Nascent Cl-ltberated by the process converts oXIdes of Iron mto soluble ferne chlonde Sandstones wIth SIlIceous cement cannot be dis aggregated wIthout damagIng the framework grams Gram SIze dIstnbutions of such rocks must be determmed under the mIcroscope usmg a calIbrated eyepIece mIcrometer 2 Sand samples must be thoroughly dned In hot air oven before slevmg To ehmmate orgamc matenals treat the sample With HzOz 3 When the ongmal sample size IS too large for sIevIng a subs ample of desired weIght must be obtamed by splIttmg The Cone-and quartenng method of sample splIttIng usually works well but a mechantcal sample splItter should preferably be used Too large a sample clogs the SIeVes, too small a sample causes delays because of the extreme care needed for collectIOn and welghmg Recommended Imtlal weights of samples to be placed on the topmost SIeVe (after Vanont et ai, 1961) Frame diameter cif sieves

Sand size

MedIUm sand size Fme sand size

8-meh (20 3 em)

3 meh(7 6 em)

2550gm 1020gm

37gm 153 gm

4 A lIst of the standard A S T M sIeves together With then mm and phI eqUivalents IS given In Table 4 6 5 The SIeves should be thoroughly cleaned and arranged m a stack from bottom to top m order of Increasmg sieve diameter In a mechamcal CRo-Tap') SIeVe shaker Ten mmutes of shakIng IS recommended normally but the tIme may be mcreased when the sample size IS large 6 All the materIals accumulated on each SIeve are to be collected and weIghed Gentle tappmg and sweepIng by a camel haIr brush may be necessary for collection of the grams 7 The results of slevmg analYSIS are to be tabulated m the manner shown m Table 4 7 Net weight of matenal correspondIng to each SIze class, weIght % and cumulatIve weIght % are to be calculated 8 The results of SIeVIng analYSIS are to be presented graphIcally and statistIcal parameters of gram SIze dlstnbutlons are to be worked out as follows

SedImentary Texture

83

Graphic Presentation ThIs Involves presentatIOn of the results of sIevmg m the form of a hIstogram, frequency polygon and cumulatIve curve (see FIg 44) The cumulatIve curve IS the best-fittmg smooth curve through the cumulatIve weIght frequency plots SuccessIve pomts on the graph must not be Jomed by straIght hnes Computation of Statistical Parameters ThIS IS done eIther (I) graphIcally or (ll) analytIcally (I) In the graphIc method percentrIes are computed from the cumulatIve curve of weIght frequency and the values obtamed are substItuted III appropnate formulae, e g , the Folk and Ward formulae (Table 4 8) to obtam the graphIc measures for mean (Mz), standard devIatIOn (0- I)' skewness ( SK\ ) and kurtosIs (Ke) The results correspondmg to Folk and Ward measures for the sample No USR 23-A are shown in table 49 (II) The analytIcal method ComputatIon of moment measures The procedures for computatIOn of moments are gIven In Table 4 10 and 4 11 The statIstical parameters of gram SIze dIstnbutIOn are obtamed as follows The fust moment (arIthmetIc mean), X =J.lI I(ftn)I'[J, where m andJdenote mldpomt of the class and weight frequency of the class respectively The second moment, 1-12 = 0- 2 = { I(fm2)-(IJm)l-1d/IJmeasures vanance (see Table 4 10) Standard devIatIon (0-) IS the square root of vanance The thIrd (113) and fourth (114) central moments are functIOns of skewness and kurtom of the dIstrIbutIOn respectIvely SImpler approxImatIOns of skewness and kurtOSIS are gIVen by a 3 and a 4 (see Table 4 11)
= {L,(m, -111)3

= \L.lm,-1-11)~

Values of )..II' cr, 4 10 - 4 11

a,3

J,} 1Ncr 3

il}! N~ and a,4 for the sample No USR 23-A are shown m Tables

Tests for NorrflOl!ty of D,strrbutlOn (I) AnalytIcal Procedure whether or not the gram-SIze frequency IS 'normaHy'

dIstnbuted can be tested eIther analytIcally or graphIcally In normal dIstnbutIOn P2 attaIlls the value of 3 and Y2 IS zero, where P2 and Y2 are as follows

P2 = 1-14 1 Il~,

= P2 -

3, where J.l4 = 4th central moment, 112 = 2nd moment = vanance Elaborate computatlons are necessary for obtammg ~2 and Y2 A sImpler method IS to ascertam whether a 3 and a 4 correspond to zero and three respectively 12

(ll) GraphIc Procedure sedlmentoiogists are accustomed to testmg the normally of

dIstnbutlon by plottmg the cumulatIve gram SIze frequency on a 'probabIlIty paper' (see FIg 4 7) A symmetrIcal ogne, whIch IS the cumulatIve curve correspondIng to a normal frequency dlstnbutlon has a perfectly Imear appearance on a plobablhtv plot The better the fit of a ~tralght lIne through the


84

plots, the closer the match to a perfectly normal distribution. When the grainsizes are plotted on a logarithmic (phi) scale, a straight line fit indicates that the distribution is log-normal. Table 4.6: Specifications of standard A.ST.M sieves

A.S.T.M

Sieve upening (mm)

Phi size

Mesh No.

V2 scale

(¢)

Nomenclature

Pebble (-2 to -6 phi)

8 10

4.00 3.36 2.83 2.38 2.00

-2.00 -1.75 -1.50 -1.25 -1.00

12 14 16 18

1.68 1.41 1.19 1.00

-0.75 -0.50 -0.25 0.00

20 25 30 35

0.841 0.707 0.595 0.500

0.25 0.50 0.75 1.00

40 45 50 60

0.420 0.354 0.297 0.250

125 1.50 1.75 2.00

70 80 100

0.210 0.177 0.149

2.25 2.50 2.75

120

0.125

3.00

140 170 200 230

0.105 0.088 0.074 0.063

3.25 3.50 3.75 4.00

270 325

0.053 0.044

4.25 4.50

5 6

7

Granule

V. Coarse Sand

Coarse Sand

Medium Sand

Fine Sand

V Fine Sand

Silt

85

Sedimentary Texture

Table 4.7: Results of sieving analysIs. Sample No. USR 23-A. Initial weight of sample = 50.292 gm ASTM

No

SIeve apenmg (mm)

(
Mid pamt (
10 14 18 25 35 45 60 80 120 170 230

PAN

>200 141 200 141 100 100 071 071 050 050 035 025 035 025 0177 0177 o 125 o 125 0088 0062 0088 <0062

< -100 - 1 00 - 0 50 - 0 50 000 000 050 a 50 100 100 1 50 150 200 200 250 250 300 300 350 350 <100 >400

- 0 75 - 0 25 025 075 125 1 75 225 275 325 375

Gross WeIght Empty (wt) watchgluss f (gm) (gm) wt (gm)

30950 18721 37015 36013 39562 38622 38959 31563 15201 14794 14770 14767

29526 14763 29526 29526 29526 29526 29526 29526 14763 14763 14763 14763

1424 3958 7489 6487 10 036 9096

9433 2037 0438 0031 0007 0004

Weight Cumulative ( %) weight

frequency 2823 7847 14847 12 861 19897 18033 18 701 4039 0868 0061 0014 0008

Total weIght 50440 gm IllItIaI weIght of sample 50292 gm Gam In weIght o 148gm Error = + 0 294%

Table 4.8: Folk and Ward (1957) grain-size measures Parameter

GraphIC mean InclusIve graphIc standard deVIatIOn

Inclusive graphic skewness

Graphic kurtOSIs

Fonnula

(M z ) = ~16 + ~jO + ~84 (0")I = ~84 4-~16 +~ 66 (SKI) = ($84 + ~16

- 2~50) + ($95 + ~5 - 24>50) 2( $84 - ~16) 2( $95 - ~5)

2823 10670 25517 38378 58275 76308

95009 99048 99916 99977 99991 99999


86

Table 4.9: Statistical parameters of grain-size dlstnbution. Sample No USA 23-A Standard deviatIOn (i/J)

Mean (i/J)

Skewnes~

KurtoSIS

Graphic measureo (Folk and Ward 1957)

ML

0704 0865

cr, SKI

- 0172 0965

KG Moment measures f1, cr

0777 0860 - 0 095

u3 u4

2133

Table 4.10: Computation of first and second moments. Sample No. USR 23-A

SIze class

MIdpOInt

m, (
(
WeIght frequmLJ

t

< -100

J,m,

tm;

(l 424)

- 1 00

-050

- 075

3958

- 2968

2226

- a 50 a 00 a 50

000

- 025

7489

- 1 872

0468

050

025

6487

1622

0406

1 00

075

10036

7527

5645

100

1 50

125

9096

11 370

14212

1 50

200

175

9433

16508

28889

200

2 50

225

2037

4583

10312

250

300

275

0438

1204

3311

300

350

325

0031

0101

0328

350

400

375

0007

0026

0098

If m =38.101

L:./ m 2

>400

(0004)

N== 'iJ = 49012 Note Figures In parentheses are omitted

In

computatIOn

First moment (anthmetlc mean) f-il = L: (/in)/ LJ =38101/49012 = a 777 ~ Second moment (vanance) fl2:: a 2 L (fin2) - (ifin) fll} 1"'Lj= a 740 ~ Standard deviatIOn cr:: 0 860
={

=65895

{I/>}

100 a 50 000 a 50 100 1 50 200 250 300 350

>

4 00

-0 50 000 050 100 150 200 250 300 350 4 00 N = 'If. = 49 012

3958 7489 6487 10 036 9096 9433 2037 0438 0031 0007 (0 004)

(1 424)

!,

'Weight Frequency'

-1527 -1027 -0 527 -0 027 0437 0973 1473 1 973 2473 2973

(m,- ~l)

J;}/(N ( 4)

U4

= {I (m,- lil)4

= {I (m, - ~1)3 1;1/CN CJ3)

u3

= 2 133

=- 0 095

Note Figures In parentheses are omitted In computatIOn ComputatIOn of u3 and u4

-075 -0 25 025 075 125 1 75 225 275 325 375

m, {I/>}

< -100

MIdpoInt

SIze class

-3560 -1083 -0 146 0000 0106 0921 3 196 7680 15 124 26277

(m, - ~1)3

I(m, - ~1)3 =-2970

-14 093 -8 112 -0 949 0000 0963 8689 6510 3364 0469 0184

(m'-~1)3 1;

Table 4.11: Computation of u3 and u4. Sample No. USR 23-A

1;

5437 1 112 0077 0000 0050 0896 4708 15 153 37402 78123

(m,- ~1)4

i,

L:(m, - ~l)'i = 57 192

21 519 8331 a 500 0000 0455 8455 9589 6637 1 159 0547

(m, - ~1)4

1;

en (1)

-..j

0>

(1)

...C-

~ x

'<

iii ...

::l

3(1)

a.

5 HYDRAULICS, SEDIMENT TRANSPORTATION AND STRUCTURES OF MECHANICAL ORIGIN

HYDRAULICS OF OPEN CHANNEL FLOWS Runnmg water transports much of the sediments m nature To follow the process of sedIment transportatIon therefore, one must have a clear Idea of the dynamICs of flowmg water The mechamstIl of sediment transportatlOn by wmd IS also comparable m many respects to that of runmng water The baSIC pnnclples of hydrauhcs are dIscussed In thIs sectIOn to provIde an mSIght mto the mechamsm of sedIment transportatIOn under aqueous and eohan COndItions The most fundamental relatIOnshIp between water dIscharge and flow VelOCIty In an open channel IS given by the continUIty equation

Q=wdv::::Av where Q = total flow dIscharge, w = channel wIdth, d = water depth, v = flow velocIty, A == cross-sectlOnal area of the channel It follows from thIS equatiOn that dIscharge and water level remammg constant, the velOCIty of flow through an open channel vanes mversely as the channel WIdth All open channel flows, mcludmg natural streams, are guIded by thIS fundamental relatIOnshIp An Important concept m fluvIal hydrology IS hydraultc radIUS, whIch IS obtamed by dIvldmg the cross-sectIonal area of the channel by the wetted penmeter The phYSIcal state of flow through an open channel IS controlled by several factors, of whIch flow velOCIty and flUId VISCOSIty are more Important The term VISCOSIty (also called molecular or dynamIC VISCOSIty) mdicates the rate of deformatIon (du/dy) a flUId WIll undergo under a gIven stress (-r) In the equatIOn 1: = 11 (dul dy), ).lIS the coefficIent of molecular VISCOSIty The dImensIons of molecular VISCOSIty are ML III In Sl umts It IS kg/tIl s or (N sl m2 ) It's umt IS pOIse 11 IS Inversely proportional to temperature The recIprocal of VISCOSIty Isfluldltythat IS, VISCOSIty decreases (and flUIdIty mcreases) wIth Illcrease of temperature At 20°C the VISCOSity of watens 0 0 I pOise Of the common flUIds, glycenne IS most VISCOUS (nearly a thousand tImes as VISCOUS as water)

Physical States of Flow WIthIn a body of flUId, a velOCIty gradIent may generate due to gravlty It may also be produced by an externally applIed pressure Such flows are calledglavlly waves DensIty differences withm a body of flUId caused by change'> III sedIment concentratIon or temperature vanatlon may also produce a flow Such currents are called dmslty ClI/ 1 ents 88

Hydraulics, Sediment Transportation and Structures of Mechanical Orgin

89

When the resistance offered by viscosity of the medium is high compared to the flow velocity, the fluid particles move in straight, parallel paths in the form of laminae. Such a flow is called laminar flow. The resistance offered by fluid viscosity is overcome when the flow velocity increases. The fluid particles under such a condition move in a haphazard fashion. Multidimensional secondary motions are superimposed on the primary flows and eddies generate (Fig. 5.1). The flow at this stage is called a turbulent flow. Osborne Reynolds expressed the relative effects of flow velocity and viscosity on the physical state of flow in 1883 by the following equation: R = v.l Iv where v = mean flow velocity, I = a linear measure like depth of water, grain-size, hydraulic radius, v = kinematic viscosity of the fluid. Kinematic viscosity is obtained by dividing the coefficient of viscosity (ll) by density (p). Turbulent

Laminar Flow

Fig. 5.1: Laminar and turbulent flows over a fine-grained bed (based on Fritz and Moore (1988). With Permission of W. J. Fritz.

Reynolds Number, a dimensionless parameter, is an expression for the balance between two contrasting forces acting within the fluid. The inertial forces shown in the numerator allow the fluid mass to accelerate, while the viscous force in the denominator resists deformation. For water at 20°C the changeover from laminar to turbulent flow takes place at the critical Reynolds number of 500. This changeover, however, is not sharp. Even when the temperature is constant, a zone of transition marks the conversion from laminar to turbulent flow (Fig. 5.2, also see Chow 1959). While the relationship between inertial and viscous forces determines whether a flow will be laminar or turbulent, the ratio between the inertial and gravity forces determines whether the flow will be tranquil (subcritical) or rapid (supercritical). The following equation named after Froude (pronounced frood) shows the relative effects of velocity and gravity waves in determining flow condition:

F=v/~ where v = flow velocity, d = water depth, g = acceleration due to gravity. F a dimensionless parameter, is called the Froude Number. When flow velocity equals the velocity of propagation of the gravity wave, that is, F = I, the flow is said to be critical.


90

At F < 1, the flow is subcritical (also called tranquil). At F> 1 the flow is rapid or supercritical. In the subcritical stage, the water level is high but the flow velocity is low. This is called the Lower Flow Regime of the stream. In the supercritical stage, the water level is low but the flow velocity is high. This is the Upper Flow Regime of the stream. The changeover from higher to lower regime is often abrupt. When it takes place, some of the kinetic energy of the supercritical flow is suddenly transformed into turbulent energy and rollers appear within the flow. The phenomenon is known as hydraulic jump. The fields of critical Reynolds Number and critical Proude Number might overlap in nature in spite of the fact that Reynolds and Froude Numbers are fundamentally different (see Pig. 5.2 stippled field).

40

s

10

u .........

-S 4.0 0fl)

Q

1.0

0.4

4.0 10 40 Velocity (cm/sec)

100

400

1000

Fig. 5.2: Hydraulic conditions at various combinations of water depth, flow velocity and temperature (after Sundborg 1956). With permission of the author.

Structures of Turbulent Flow Turbulent flows are characterized by eddies having vortices. Fluid eddies produced by the vortices are responsible for redistributing mass as well as momentum within the flow. Turbulent eddies are three-dimensional. Hence, the flow, while moving essentially in the x-direction, has y- and z-components also. Researches conducted over the last 30 years or so have provided insights into the physical nature of turbulent motion. It is now known that a turbulent flow is characterized by 'lumps' of fluids, which move a) from outer to inner flow region ("sweeps"), and b) from inner to outer flow region ("bursts "). These movements take place in the horizontal (x, y) plane, at the bottom of


91

the flow. Near the base of the vertical (x, z) plane slow- and fast-moving streaks ("lanes") of fluid alternate. Higher up they lose their identity. In three-dimensions, the vortex structures have the shapes of hairpins whose legs are constituted of low-speed streaks. The hairpin vortex eventually give rise to longItudinal ridges and generate bed defects. Sweeps amplify bed defect heights. ThIs IS followed by flow separation, which generates downstream erosion and initiates ripple propagation on the sediment at the bed (Best 1992, see Fig. 5.3). Spacing of these streaks, and also, the rate of bursting of the lowspeed streak decrease with increase of shear velocity. The picture IS ever changing because new sreaks are generated and regenerated across the flow Cogent accounts of these recent findings can be found in Leeder 1999. Mazumder, 2000. Primary hairpin (A)

(B)

. (C)

.. '.' .......

"

.,','. ,

..

' ,"

,':.'

..

"

.. , .......... . . ,

(D)

,

Fig. 5.3: Turbulent boundary layer structures leading to sediment entrainment by flow. (A) Hairpin structures and low speed streaks generate at the boundary layer. (8) Multiple streaks and sweeps generate ridges parallel to flow. (C) Bed defects are produced. (D) Flow separation causes downstream erosion and ripple propagation. Modified and redrawn from Best, J. on the entrainment of sediment and initiation of bed defects: insights from recent developments within turbulent boundary layer research. Sedimentology(1992) 39,797-811. Publisher, lntema\ional Association of Sedimentologists (With permission of Blackwell Publishing).

SEDIMENT TRANSPORTATION When the velocity of water flowing over a bed of non-cohesive sediment exceeds a certain threshold value, the grains are put into motion (entrained). At this stage, the

IntroductIon to SedImentology

92

flow overcomes the resistance offered by the gram due to Its mass, coheSIon and other forces Properties like shape and pOSition of centre of graVIty of the gram play Important roles m determmIng the degree of resistance offered A diagram complied by HJulstrom (1935) revealed the relatIOnshIp between crttlcal erosIOn veloCIty and gram-sIze (FIg 5 4) The field above the curve mdicates the conditIOn SUItable for entraInment of grams It IS clear from these dlagra:ns that the particles most eaSIly eroded from a bed are of finesand SIze (0 3 to 0 6 mm) 300 EROSION

100

r TRANSPORTATION

c[) o

SEDIMENTATION

;E

OIL-~~UU~~~~~-L~~~~~~~~~~~--~~

0001

001

o1

Gram SIze (mm)

I

10

100

500

~

Fig. 5.4: Relationship between flow velOCity (at a height of 1 m above bed), graln,slze and eroslon·transportation-deposltlon of sediments (after HJulstrom 1935)

PartIcles coarser than fine sand are dIfficult to erode because of theIr weIght On the other hand, still finer grams of silt and clay SIze, resist erOSIOn due to theu smoothness and coheSIOn The veloCity needed for erodmg a partIcle from a bed IS not the same as that at whIch the same gram, If already In motion, comes to rest The erodlllg veloCity must be decreased by about 30% to cause depOSitIOn of the coarse grams Hence, the cntIcal erOSIOn VelOCIty (CEV) m the HJulstrom diagram IS a zone, not a lme Clay and fine sIlt, once entraIned, may remam m suspensIOn even at a low velOCIty Curve B III thiS dIagram corresponds to the lowest transportIng velOCIty HJulstrom's studIes, although of fundamental Importance, have theIr limitations For example, later workers have demonstrated that for the very coarse sediments the HJulstrom curve underpredicts the entrarnment velOCity (Novak 1973) The mfluence of the drag force (,*) of the flUId on the sedImentary partIcle, and the leslstance offered by the gram m terms of turbulence CR.) at the boundary on the cntlcal erO~lOn velOCity was expenmentally determmed by ShIelds m 1936 (Fig 5 5) Shields' curve IS valId for grams transported by water as well as WInd To move from water to


93

air one only needs to change the parameters of the equations. Experiments conducted by Bagnold (1954) showed that in the case of wind also fine sand is most easily eroded. The surface provided by silt and clay is aerodynamically smooth and hence difficult to erode. Experiments have shown that for water as well as wind a certain amount of form resistance is essential for initiation of grain movement. This resistance is provided either by some extraneous bodies lying on the bed surface, or by some irregularities projecting out of the bed into the laminar sublayer. These findings have given rise to the concepts of smooth and rough boundaries. 10° r-.-rr~~-r"~mr-.-rTnmm--,,-nTm~~TTnm~-rIlTn~

Modified by Miller et al. (1977)

{p

o f.'

----I ;-

Shields (1936)

10- 1

v, ;--...'

.

f.'

10

2

10

2

Fig. 5.5: Curves for sediment transportation as determined (A) by Shields 1936, and (B) modified and extended by Miller et al. 1977. v. == shear velocity, d == particle diameter, v = kinematic viscosity, y = specific gravity of the fluid, "'Is == specific gravity of the particle, '0 =average boundary shear. With permission of International Association of Sedimentologists.

Smooth boundaries develop when the sedimentary particles of a bed lie wholly concealed within the viscous sublayer. On the other hand, rough boundaries may be produced when the particles are not fully concealed. In the latter case the particles projecting out into the laminar sublayer may shed off eddies whose sizes are directly proportional to those of the particles. In the intermediate stage, when the grains are only partly concealed, the boundary is called transitional. The flow velocity immediately above a sediment bed is retarded due to friction at the bed surface. The flow loses a major part of its kinetic energy here. The thin layer of fluid adjacent to the bed where this retardation takes place is called the boundary layer. Within this layer the flow can be either laminar or turbulent. Even when the flow is turbulent, a very thin fIlm, immediately in contact with the bed, shClws laminar motion. This laminar sublayer generates over extremely smooth beds offering no resistance to flow. Close to the wall of the channel also there is a layer of liquid, which retards movement due its surface attraction. This is called the adsorbed layer. The turbulent boundary layer can be divided into two distinct zones: an 'inner' (lower) zone, close to the bed and an 'outer' (upper) zone extending up the free flow surface. The former, which is the zone of vigorous turbulent activity, contains the lowand high-speed streaks of fluid. When the shear velocity is high, the low-speed streaks


94

are hfted up to the base of the upper boundary The 'burstcydes', Initiated at the lower boundary of the overlymg layer, dissipate, whde beIng transmItted through the upper layer, and may show up at the surface as bOlls The nature of form resistance provided by a rough bed IS given by a number of well estabhshed equations given below (Rouse 1938, p 279, Chow 1959, p 99) The Chezy equation,

v = CJRh S

T h e M annmg equation, V =

The Darcy equation,

JJ

[I 49 (Rh)213 SII2J / n =Rh/6 138n,

where V = umform flow velOCity, Rh = hydrauhc radIUS, S = channel gradient, C = Chezy fnctIOn coeffiCIent, n = Mannmg roughness coeffiCIent, f = Darcy S fnction factor The expression for Mannmg equatIOn gIven above IS m f p s units In metTle umts the constant 1 49 IS replaced by umty The velOClty profile of the body of water flowmg through an open channel IS loganthmic with respect to hetght except at the very bottom where It IS lmear (Fig 5 6) ThiS velOCity profile, deduced from the stmrlanty theorem of von Karman vanes, dependmg on whether the bed concerned IS smooth or rough The constants In the Karman equations were expenmentaUy determmed by later workers After msertlllg these constants the well-known Prandtl-Karman velOCity dlstnbutlOn equatIOns read as follows (Sundborg 1956)

y

d

Lmear

Fig. 5.6: VelOCity dlstrlbullon over a non-coheSive sand bed

Hydraulics, SedIment TransportatIon and Structures of Mechanical Orgm u/u.

= 5 7510g 1O (zu*/v) + 5 5

for smooth boundary

u/u*

= 5 7510g 10 (zIK) + 85

for rough boundary

95

where U IS the average veloClty at a dIstance z from the boundary surface, u* IS the shear stress veloClty v IS the kmematlc VISCOSIty, and K IS the dIameter of sand gram on a sandy surface

Sediment Transportation by Water and Wind Once entramed, runnmg water m four dIfferent ways transports sedIments shdmg, rollmg, saltatIon and suspenSlOn The first two, confined to the stream bed, contnbute to what IS commonly called the bed load or tractIOn load In natural streams rollmg IS more Important than shdmg A gram rolls forward m response to the Impact of flUld (,velOCIty pressure') agamst It Larger partlcles offenng greater surface areas move more easIly than smaller grams ThIS movement IS opposed by the VISCOSIty pressure whose magmtude depends on the fluId's coefficIent of VISCOSIty, densIty and veloCIty The total drag (Fc) IS due to the pressure dIfference between the two forces In an accelerated flow, the Bernoulh law demands a reductIOn of pressure transverse to the dlrectlOn of flUId motion ThIS causes a reductlOn of pressure on the upper surfaces of the grams movmg m the flow A gram IS hfted up mto the flow whenever the force created by the pressure difference between ItS upper and the lower surfaces exceeds ItS submerged weIght The upward component of the turbulent flow IS also an addItIonal aId to the process of hftmg In fact, for grams of sand and s11t SIze, turbulence mIght be the controllmg factor, because the effect of BernoullI hft on such grams IS neghgible Tangentlal forces actmg on a partIcle m motion may occasIOnally force It to leap forward mto a state of temporary suspensIOn untIl the force of gravIty bnngs It down to the bed The whole process IS then repeated and the partIcle moves forward m a senes of Jumps, constItutmg what IS called the saltatIon load (Latm saltare-to dance, see FIg 5 7) Saltatmg grams follow low, smooth traJectones and are governed largely by ballastlc forces The veloClty of a partIcle dunng saltatIOn IS less than that of water m motIon (FranCIS 1973) Much of the sand SIze particles IS transported by wmd m saltatIOn while the coarser grams are left behmd as lag depOSIts The saltatmg grams Impmgmg on the sand surface at low angles put mto motlOn the grams, whIch are normally, too large to be entramed Only the upper few mIlhmetres of a sand bed are affected by this surface creep but the saltatmg grams may themselves be raIsed up to a heIght of one metre or so At hIgher levels, finer dust constItuted of SIlt and clay size partIcles, moves m suspenSIOn, bemg supported by the upward component of turbulence ThIS IS true for wmd as well as water m motlOn Wmdblown dust extends to many kilometres above desert sand (FIg 5 8) The turbulence m case of wmd IS particularly high because of the low VISCOSIty of aIr On the contrary, the suspensIOn load m water rarely extends to more than a few metres above the bed Washload ('dustload' m case arr flows) IS the name aSSIgned to the very fine (clay SIze) partlcles which are transported 10 permanent suspension (Leeder 1999, p 134)


96

~urlace

p FluId densIty

1 h

U

)

FluId densIty

flow depth

1

2

Bed slope Gravitational mass (mg)

~

How grains move I Gram rolling 2 Gram saltation 3 Gram suspension

Fig. 5.7: The physical conditions for grain entramment, saltation and suspension (from Leeder 1999) With permission of Blackwell SCience Ltd.

SuspensIOn - kms Slit-clay

Sand

Granule Fig. 5.8: Eolian sediment transportation (based on Fritz and Moore 1988) With permisSion of W.J Fritz.

Estimation of Bed Load

An early attempt at estimatIon of bed load was made by H A EmsteIn In 1942 In the modified verSIon of hIS equatIon, Emstem (1950) assumed that particle movement IS


97

mltlated when the hydrodynamic lIft force exceeds the submerged weight of the particle He further assumed that the probablltty of thIS partIcle bemg redeposIted IS equal at all pomts of the bed and the average distance travelled by the particle as bed load IS constant between consecutIve pomts of depOSitIOn Emstem's assumptIon that this distance (about 100 tImes the diameter of the partIcle) IS mdependent of the rate of transport, bed- and flow conditIOns, was questIoned by later workers Emstem's bed-load equatIOn relates the rate of bed-load transport (<1>*) to the propertIes of the gram and the flow (\jf*) A correctIOn factor (8) was mtroduced m 1953 by Emstem and ChIen for materials whIch are fme or non-ulllform (see Grade and Ranga RaJu 1977, p 145 for detaIled dIscussIOns) Bed load equatIons were later developed by many authors Shen (1978) revIewed these equatIOns and recommended the use of Emstem's methods as modIfied by Colby and Hembree (1955) for estImatIOn of bed and suspended loads relatIve to some measured data when these are avaIlable In India, Goswaml (1988) applted as many as eight dIfferent formulae to estImate the bed-load transportatIon rate m the Brahmaputra RIver and obtamed WIdely varymg results, suggestmg that no one of these equatIOns can fully accommodate all the natural vanables Laursen's (1958) equatIOn, accordIng to GoswamI, proVides the most acceptable result m the case of the Brahmaputra

Estimation of Suspension Load A suspension (suspended) load IS defined as one m which the weight of the suspended particles IS contmuously supported by the flUId The force necessary to keep the particles heaVIer than water or wmd m suspensIOn IS proVIded by the vertIcal component of turbulence As thiS IS a fluctuatmg phenomenon, attempts to compute m absolute terms the amount of suspenSIOn load m a turbulent flow have led to extraordmanly complex exerClses On the other hand, equatIons have been developed for estimatIon of relatIve concentratIOn of suspended load m open channel flows (Rouse 1938, p 341)

Cy/Ca = [{(d-y)/y} {a/(d-a)}]Z where z = v/xu* Ca and Cyare sedIment concentratIOns at distances a and y above the bed respectIvely, d IS total depth of water, Vs IS the settlmg veloCity of the gram, u. IS the shear veloCity X, the von Karman constant, IS 0 40 A SImpler expression of the exponent z has been given by Sundborg (1956, p 214) Z = 6 25c In (dlzo)/ Umax where Umax IS maXImum velOCIty m profile, CIS settlmg VelOCIty of the gram, Zo IS a measure of the roughness of the bed gIven by ksl30, where ks, the representatIve gram dIameter of the bed matenal, IS gIven by that SIeve size of which 65% of the mIXture (by weight) IS finer The equatIon proVIdes a SImple method for estImatIOn of suspenSIon concentratlOn at any level (C), prOVIded the concentratIon at the reference level (CJ IS known Sundborg (1956) constructed several modIfied curves for cntIcal erOSIOn veloCIty, cessatIOn of gram movement and relatIve concentratIon m suspensIon LJunggren and Sundborg (1968) also combmed the curves for transportatIon of mmerals of different densIties m a composite dIagram (Fig 5 9) It must be noted that all these dIagrams are based on expenments WIth (gram-SIze WIse) homogeneous matenals In natural

10

20

o

N o

o

40

60

100

I

o

o

......

:&'

0

-~----

or;; ~

q

~

.. ,'

CJrain dianneterin nnnn

ci

......

~o

--~ -~--~--_:---ifj------p,(j ....: .......':.'.' .• ' ---Ib----""---- 7'~'-; Sf $---... : ~ ....;.. ''

________

»

q

......

'O-V 'f,.\~"-'1

~<.?J.-V?'¥O

>

q

o

o

oN

Fig.S.9: Relationship between grain-size. flow velocity and transportion of minerals of different specific gravities. Stippled area indicates environment suitable for heavy mineral enrichment (after Ljunggren and Sundborg 1968). With permission of A Sundborg,

;;--

V

0

'u

C

.S

u

----E

rJJ

V

u

200

400

CD

'<

0' IC

o

3' m ::l ,..

Q.

::l ,.. o en /D

cr

a. .:= o ,..

a a

0)

Hydraulics, Sediment Transportation and Structures of Mechanical Orgm

99

conditIOns, the process of sediment transportatIOn IS much more complex ThIs IS so because a large range of gram SIze IS mvolved m nature Only a few attempts have been made so far to study the mechamsm of gram sortmg m heterogeneous sedIment loads (Sengupta, 1975, 1979, Sengupta et al 1991, Ghosh et al 1981, and 1986, Sengupta et al 1999)

Capacity and Competency Gllbert (1914) defined the total load a stream can transport as capacIty and the size of particles It can carry as competency In case of transportation by suspensIOn current, capacity IS expressed m terms of grams per htre of suspension Competency IS represented by the coarsest gram (or by the SIze of the coarsest 1% of the gram-size dIStributIOn) transported Expenments have shown that the ratios of grams per litre m suspensIon at two dIfferent flow velOCIties vary With the size class mvolved Hence a current's capacity to carry a load m suspensIOn depends on the coarsest size It can carry, that IS, on ItS competency Grams belongmg to a particular size class m suspensIOn are deposited when the capaCity of the current to hold matenals of that size class m suspension IS exceeded (Kuenen and Sengupta, 1970)

BED CONFIGURATION DURING SEDIMENT TRANSPORTATION Water flowmg over a plane, non-coheSive sedIment surface throws It mto a senes of rhythmiC patterns called bedforms These are mitiated at pomts of bed IrregularIties by processes shown m FIg 5 3 (Best, 1992) The process of evolutIOn of a varIety of bedforms, startmg from a Simple, plane bed was expenmentally mvesttgated by a number of researchers The results of one such plOneenng study by Simons, RIchardson and Nordm (1965, see FIg 5 10) demonstrate the mfluence of gram-size and flow-mtenslty on the morphology and magmtude of bedforms Stream power eto v) wAs used m thIS dIagram mstead of velOCIty because the latter vanes as loganthm of height to v, whIch has the dimenSion of power (work done over umt area m umt time) represents the erosIVe power of flow on the bed Fall dIameter, which mdlcates the hydraulic behavIOur of a gram, IS defined as the diameter of a sphere havmg a speCific gravity of 2 65 and the same termmal umform settlmg velOCIty as the particle concerned The flows responsible for generation of bedforms of dIfferent types were dlVlded lUtO three categones accordmg to Froude number lower flow regime (F < 1), transitIOnal flow regIme (F - 1), and upper flow regime (F> 1) The first to develop m the lower flow regime are the small sand rIpples With mcreasmg stream power and fall diameter of the bed matenal, larger bedforms (dunes) appear Long, flat sand waves mark the transItional zone (F - 1) With lllcreasmg steam power, these are replaced by plane beds of the upper flow regime and eventually by antldunes At thiS stage, the water waves break m a duectlon opposite to the flow and the sedIment transport drrectIOn IS reversed The antidunes are m phase With the waves generated at the water surface, whereas the npples dnd dunes m the lower flow regIme are not More recently low-amphtude bedforms have been discovered on the upper-stage plane beds These are out of phase with the overlymg flows (Leeder 1999, P 156)


100

Upper Flow Regime (F>l)

4.0

Antidunes

Critical Flow (F

1.0

= 1)

I;> ~

Dunes

Lower regime / plane beds I

o

0.2

0.4

0.6

~ ~

0.8

1.0

1.2

MEDIAN FALL DIAMETER (mm) Fig. 5.10: Bedforms generated in experimental channels at various combinations of fall diameter and stream power (after Simons. Richardson and Nordin 1965).

Repetition of these studies in different laboratories during the last three decades has led to modification and refinement of the original Simon curve. One of the later results, based on the experiments conducted by Southard and Boguchwal (1990) is shown in Fig. 5.11. Grain-size plays an important role in the passage of one bedform to the other. Only when the grains are of medium sand size do all the four different types of bedforms appear at increasing velocities. When the grain-size is very small « 0.15 mm), ripples grade directly into upper plane beds without passing through an intervening dune stage. Similarly, small ripples do not generate at all in coarse sand. Another important factor in the development of bedform IS depth of water. A minimum

Hydraulics, Sediment Transportation and Structures of Mechanical Orgln

101

depth of about 4 cm of water IS essentIal for the appearance of all the four dIfferent types of bedforms The mfluence of the three dIfferent parameters, gram SIze, flow velOCIty, and water depth on the nature of bedforms IS shown In a three dImenSIOnal dIagram (FIg 5 12) A SImIlar dIagram was also constructed by Rubm and McCulloch 1980

,,-..,

20

Antidunes

Fr = 10 Fr = 0 8

r/J

'-

S

'-" 0

:-,

:5

upp~~~o,,~

» .....

U 0

~

08

~

Dunes

0

c;:: ~

06

il6.rl;

CIl

E ..... ~

v

3D

abrUpt

:Ot

2D

04

~::3

RJpples

No Movement

0<

v

CJ 0

0

02

.......

o1 10° C - eqUIvalent sedIment SIze DIO (rum) Fig. 5.11: SchematiC representation of stability fields for bedforms of different types at various combinations of flow velOCity and grain-size for flow depths = 0 25o 40 m Froude Number IS Indicated by Fr (From Southard and Boguchwal 1990) With permission of SEPM (Society for Sedimentary Geology)

IntroductIon of the concept of flow regIme III the study of bedforms was prompted by the Idea that an assemblage of bedforms Imphes a state of hydrauhc eqUIhbnum that IS hard to define A number of vanables, not only fall dIameter and stream power, but even parameters lIke temperature and the amount of clay suspenSIOn, WhICh affect VISCOSIty, play Important roles III defimng thiS eqUIlIbnum The concept of flow regIme III a closed system lIke a laboratory channel agam, IS not the same as that appbcable to nature In the latter case, vanables lIke rates of sediment supply and deposltlOn, and also non-avaIlabilIty of the tIme needed for attaInmg hydrauhc eqUlhbnum, may cause compllcatlOns whlch are not faced m laboratory experIments (Blatt, MIddleton and Murray 1972, p 123-124) The SImplest type of repetItIve bedform, generated transverse to umdIrectIOnal flows, IS a sand npple (Fig 5 13) The steeper face of a npple dlppIllg III the downcurrent


102

direction is called the lee face. The gently sloping face dipping in the opposite direction is the stoss. The crest of the ripple is named the summit point. The vertical height of the crest above the base is the height (H) of !:he ripple. The horizontal distance from one crest to the next is the length or wavelength of the ripple (L or A). The ripple index, also called the vertical index, is given by the ratio of length to height (L/H) .

.,' .::': '.: . . :.'. '.','.'.:-:: d~~~·:;·j~· ;.;~.: .: .... :. ..

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.

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.

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.

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f0'?J

..." 0.6

S 10

O. '. e
Fig. 5.12: A three-dimensional representation of stability fields of bedforms· at various combinations of water depth, flow velocity and grain-size.

The stream flow lines associated with an asymmetric sand ripple are shown in Fig, 5.13. Erosion of sediments from the stoss face of the ripple and deposition of these sediments on the lee side causes the ripple to 'migrate' downstream. The internal stratification developed out of this process produces cross-stratification on the lee face. On the lee side of the ripple, immediately below the ripple crest, a change in the direction of flow causes instability (termed Kelvin-Helmholtz instability) and develops a system of vortices rotating on horizontal axes. This is an important mixing mechanism.

® Flow

;.

Wavelength (I.) or Length (L)

~

'Q) :::r::

Brink

~

Trough

Backflow rippJes

........

{j

Fig. 5.13: (A) Morphology of an asymmetric san(}'~iPPle. low lines related to an asymmetric sand ripple migrating by stosS"'side erosion and lee-side deposifon.

I(

o

flow

Summit

::I

W

o

~

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!!!..

(i'

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I»

::r

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::::I

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iii c:

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104

IntroductIon to Sedimentology

A" the pressure mcreases m the down flow dIrectIOn the layer of flUId at the bottom IS forced upstream by a process called flow separatlO(l The process generates back/low rtpples 10 front of the advancmg lee The pomt where flow <;eparatlon IS Imtlated IS the brznk poznt The pomt of flow-reattachment IS the toF' pOint of the npple RIpples and dunes makmg sharp angles With the hOrizontal are called angular bedfonns Laboratory expenments have shown that largely the sediments on the lee sIdes of npples control the stablhty of angular bedforms When gram-size and flow velOCIty condltlOns cause the ~edlment to avalanche near the ripple crest, to make a concave up profile, an angular bedform pattern IS m;1mtamed Flat beds generate when the coarser grams are carried to a greater dIstance at hIgh flow veloCIty Under thIS condItion, the lee face becomes convex and the bnnk pomt vamshes Coarse sands (d > 07 mm) do not npple because they cannot mamtam a shP face (Hand and Bartberger 1988) In fact, sand coarser than 0 7 mm IS beheved not to nyple at all (Leeder 1999, P 150)

Classification of Ripples The CriterIa commonly used for claSSIfication of sand npples are shape and magnItude The former IS defmed by crestlme and the latter, by wilvelength and heIght Four dIfferent shapes of ripples were recogmzed by Allen (1968) Tl1ese are straight, ImguOld, catenary, and lunate (FIg 5 14) Lmguold ripples are arratlg ed m en echelon (out of-phase) patterns When the npples are m phase, the pattern IS called cuspate These npple patterns were correlated WIth mcreasmg flow veloCitY and decreasmg water depth (FIg 5 15) More recent studIes have shown that m very (Jne sand (medIan dIameter = 0095 mm) hnguoid rIpples havmg average heIght of 13 1 mm and wavelength of 115 7 mm develop from mClplent through straIght and smuou s forms prOVIded suffiCIent tIme IS allowed The time needed for development of these eqUIlIbrium npples IS related to an Inverse power of flow veloClty and ranges from several mInutes to more than hundreds of hours (Baas 1994)

STRAIGHT

FLOWe::>

FIg 514 : ClaSSification of ripple forms according to crestline (after Allen 1968)


o

Depth decrea<;es VelocIty Increases

y

)

I

I

Symmetnc StraIght

o

..-----l

LmguOld

Depth decreases VelOCIty mcreases

y

Asymmetnc

~

Smuous

105

)

B

StraIght

Smuous

Catenary

Lunate

c StraIght

Lmguoid

Fig. 5.15: Influence of flow velOCity and/or water depth on ripple form A:: small scale npples B = large scale ripples, C :: small scale ripples transverse to flow Lee Sides are stippled (from Allen 1968a with permission)

Magmtude-wise npples have been tradItIOnally claSSIfied mto npples, meganpples, dunes and sandwaves, but lack of umformity m the defimtlon of these terms led to a good deal of confusIOn m the past Attt'mpts were made from tIme to tIme to reVIew the bedform nomenclature to resolve the confusIOn Accordmg to current thlllkmg, bedforms should be claSSIfied accordIng to descnptlve parameters, whIch are objectIvely defmable, easIly measurable and have genetIC ImpitcatlOns The classes should be mutually exclUSIve and theIr names should unambIguously define the nature of the bedform (Ashley 1990)


106

A committee appointed by the SEPM Society for Sedimentary Geology reviewed the problem of classification of ripples in 1987 and recommended use of the term 'dune' for eolian as well as subaqueous ripples of large (L > 0.6 m) dimension. The same Committee also recommended classification of sand dunes into four groups: small (L 0.6-S.0m), medium (5.0-10.0 m), large (10-100 m)and very large (> 100m). Only two groups of shape terms were accepted by the Committee. When the dune geometry can be adequately described by a single transect parallel to the flow, that is, when the dune is straight crested, it is designated as 'two-dimensional'. Dune forms characterized by scour pits and curved lee faces are called 'three-dimensional'. The other secondary criteria recommended by the SEPM Committee are listed in Table 5.1.

=

Table 5.1: Classification scheme for subaqueous ripples and dunes (based on SEPM recommendations in Ashley 1990)

I.

Essential Characters

Ripples

Small Size: Spacing (L or A.) =0.1- a.6m 0.6-5 m -0.04m Height (H) 0.Q7S-0.4m Shape: 2- dimensionl (2-D) Straight crest line 3-dtmensional (3-D) Curved crest Ihie

n

Dunes Medium Large 5-10 m 10-100m 0.4-0.75 m 0.75-5 m

Very large >IOOm >5 m

Important Characters Superposition : simple or compound (sizes and relative orientation) Sediment characteristics (size, sorttng)

m

Useful Characters Bedform profile (stoss and lee slope lengths and angles) Full beddedness (fraction of bed covered by bedforms) Flow structure (time-velocity characteristics) Relative strengths of opposing flows Dune behaviour-migration history (vertical and horizontal accretion)

Structures Related to Cprrent Ripples As current flows the stoss side of a ripple is progressively eroded and the eroded materials continuously accumulate at the brink of the ripple. When the lee slope exceeds the angle of repose, which is about 35° for subaqueous ripples, the eroded materials avalanche down the lee slope. If new sediments are added to the system at this stage, the ripple not only migrates forward but also builds upward. The net direction of ripple movement now is described by an angle of climb, e (Fig. 5. 16A). The lamination produced by this process of accumulation is called ripple-drift cross-lamination. Jopling and Walker (1968) recognized three different types of ripple-drift cross-laminations. When the grain movement is restricted to the bed, the ripple stoss is eroded almost completely and climbing sets develop on the lee side (type-A ripple-drift, Fig. S.16). When fallouts from suspension are also involved, sinusoidal ripple lamination, consisting of continuous series of sine·wave profIles (type-B) is generated (Fig. 5.16B). Intermediate between these two is the 'type C' of Jopling and Walker (1968). The typeC ripple laminae consist of climbing sets of lee-side faces together with stoss-side laminae. Alignment of ripple crests in type-B climbing ripples may sometimes produce the illusion of cross-stratification ('pseudo stratification' ; also see Fig. 5.17).

Hydraulics, Sediment Tran sportation and Structures of Mechanical Orgin

107

Fl uw~

Pse udostrat ifica ti o n

~ Type-B (S inusoida l)

Type-A

Type-C

~ A

~ Transitional

~ ~~B

----~

Fig . 5.16 : (A) The mechanism of generat ion of rippl e drift laminatio n (based on Harms et al. 19 82). B ) Different ty pes of ripple drift laminations (based on Jopling and Walker 1968).

Fig. 5.17: Type-A and Type-B ri pple drift lamination as seen in a lacker peel of channel island deposit. Ganga River near Bhagalpur, India. Scale in cm (courtesy of A. Chakrabarti) .

108


When the de\,ositional system contains mud as well as sand, intermittent breaks in the current flow may lead to preservation of streaks of mud within the ripple troughs. The resultant structures have been termed /laser bedding (Reineck and Wunderlich 1968). With increasing proportions of mud, wary beddings are produced. When the proportion of mud is very high compared to that of sand, only isolated lentic1es of sand may be preserved within thick layers of mud, leading to what has been termed Imticular bedding (Fig. 5.18).

Flaser Bedding

Wavy Bedding

1

Lenticular Bedding

Mud

[-;--':':':':'..:.:.:.:): ::. Sand ': :: ~' ' ,

... ....

Fig. 5.18: Influence of mud and sand proportions on fiaser, wavy and lenticular beddings (after Reineck and Wunderl~ch 1968).

Contemporaneous cutting and ruling by a migrating ripple produces one of the very common sedimentary structures called cross-stratification. These structures consist of beds having a higher inclination than the principal surface of accumulation. Crossstratifications may also generate in two stages-an initial stage of scouring followed by a stage of deposition (scour-and-fill). Emptying of sediments carried by streams into a relatively quite pool of water may also produce cross-bedded sets. These cross-beds have three components: a topset lying parallel to the principal surface of accumulation, a middle part (foreset) having stratification inclined at a much higher angle than the topset, and a lower bounding surface parallel to the depositional floor (bottom set). An ideal delta depicting all the three cross-bedding components is often called a Gilberttype delta, after G.K. Gilbert who described such features from the delta of Lake Bonneville about a century ago. Cross-strata may be classified according to their magnitudes and shapes. Those measured in ranges of centimetres are called cross-beds (> 1 em), and those measured in millimetres are called cross-laminations (< I em). The shape classification uses the character


109

of the lower bounding surface (simple, planar and trough) and the shape of sets of crossstrata (tabular; lenticular and wedge) as the main criteria (McKee and Weir 1953). Several other less important criteria are also used (see Fig. 5.19 and Table 5.2). Allen (1963) proposed a very comprehensive scheme of classification based on the character of the lower bounding surface and five other morphological features (Fig. 5.20 and Table 5.3).

Coset of cross-strata

I

Simple cross-stratification

Set of cross-strata

Planar cross-stratification

Trough cross-stratification

Fig. 5.19: Classification of crOSS-bedding (after McKee and Weir 1953). Table 5.2: Classification of cross-stratification (abridged from McKee and Weir 1953)

I.

NATURE OF LOWER BOUNDING SURFACE (LBS) A. LBS - non-erosional B. LBS - planar surface of erosion C. LBS - curved surface of erosion

II.

SHAPE OF EACH SET OF CROSS-STRATA A. Sets bounded by converging surfaces B. Sets bounded by planar, parallel surfaces C. Sets bounded by planar, converging surfaces

Terminology --- Simple :-- Planar --- Trough

--- Lenticular --- Tabular --- Wedge

Note: Based on criteria I and II cross-bedding sets are termed planar-tabular, planar-wedge or troughleticular.


110

Both planar and trough types of cross·stratifications may be produced by dune migration under aqueous and eolian conditions. In tabular cross-strata the bedding surfaces are planar or parallel and the foreset laminae meet the lower bounding surface at sharp angles. In plan the traces of the laminae are straight or nearly so. These are produced by migration of two·dimensional, straight· crested dunes (Fig. 5.21 B). A though type of cross-bedding is produced by migrating, three-dimensional, sinuous· crested dunes. The set boundaries are curved in longitudinal section. The foreset laminae of trough cross·beds are also curved in plan and have a tangential relationship to the underlying erosional Surface (Fig. S.21 A). C

B

A GROUPING

D

MAGNITUDE

IiJ

Grouped

Solitary

ENVIRONMENT

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E

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P, G

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Non·erosional

Gradational

LOWER BOUNDING SURFACE

~~

H

Irregular

K

Planar

~4MJ~ ~.,- 1IIr-

L

Cylindrical

Scoop-shaped

M,N

Trough·shaped

. • /~ ~ • ANGULAR RELATION

~.:.:::- ~

...:;:;.:..:__

- ----

Concordant

p

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-_

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LITHOLOGY

.--:::::' .......

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/: ..

--...

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-.

........ -;, .. .:::---

Discordant

llomogeneous

Heterogeneous

Q

R

S

Fig. 5.20: Classification of cross·bedding (after Allen 1963). With permission of the author and Elsevier Science Publishers.

Fig. 5.21; (A) Large-scale trough-lenticular cross-bedding produced by migrating sand dunes, (8) Planar-tabular crossbedding produced by migrating sand waves (after Harms et al 1975) With permission of SEPM (Society for Sedimentary Geology) ,

±~:'·$}f::.@.;,0]f:::·;~;\:\i.··.;;:.~:.:~~!;:::?i·':l:.;~~:·f :';:::::~U'~f

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112

Table 5.3: Criteria of classification and descriptive terms applicable to cross-stratified units (Allen 1963).

Cntena

Groupmg Magmtude

Character of lower boundary

Shape of lower boundary

Relation of cross-strata to lower bondary

LIthology of cross-strata

Irregular Planar Small-scale

Cylindncal Concordant Erosional Non-erosional Scoop shaped

Homogeneous

Grouped Large-scale

Gradational Trough-shaped Discordant

Heterogeneous

Descriptive Solitary terms

The type of cross-bedding generated by the process of dune migration is largely controlled by flow velocity and the amount of suspension load available in the system. With increasing flow velocity and suspension load, a tabular cross-bedding is likely to be modified into a lenticular one with a curved, tangential contact at the base. The experiments conducted by Jopling (1965) provided insight into the mechanism of this change (see Fig. 5.22). Flow separation at the brink of an advancing dune causes the sediment load to split into two fractions: a bed load and a suspended load. The former, accumulating at the brink of the dune, avalanches down the lee slope, adding foreset laminae to the lee face. The latter, caught up in the zone of turbulent mixing, is transported further downstream, to be deposited at a distance from the lee, below the zone of mixing. When the flow velocity is high, these deposits are pushed up the lee slope by the backflow vortex, thereby modifying an angular contact into a tangential one. Backflow ripples of this type have been reported by Boersma (1967) from ancient as well as modern sediments. Velocity profile

Water surface

Zone of

Progressive -ripples

Hummocky npples

RegreSSive ripples-

Fig. 5.22: Flow over a delta (dune) shaped bedform in an experimental channel indicating the Origin of trough cross-bedding (after Jopling 1961)_

A trough-leticulartype of cross-bedding may also be generated from contemporaneous cutting and filling during the course of ripple migration. Impingement of the flow jet in front of an advancing dune may cause the bed to erode at the point of flow reattachment When the dune crest is sinuous, the process of erosion may produce

Hydraulics, Sediment TransportatIOn and Structures of Mechanical Orgln

113

a senes of spoon-shaped scours at different dl~tance~ from the advancmg dune front These are soon filled by lenticular, lee face lam mae of the migrating dune, produClllg a trough-lenticular type of cross beddmg Large scale, trough-shaped scours may also be scooped out of stream beds by eddies (called' kolks') sWHhng With semi-vertICal axes Such eddies are commonly generated at the confluence of stream channels meetmg at sharp angels The filllllg of these trough shaped scours by advancmg dune laminae may produce trough-lenticular cross-beds of large dlmenslOns Such features have been descnbed front anCient and modern sediments among others, by Harms and Fahnestock (1965), Coleman (1969) and Sengupta (1974) (see Fig 5 23B) The mternal stratIficatlOns of eoltan dunes are not exactly the same as those of subaqueous dunes Four different cohan stratificatIOns have been Identified plane bed lamlnatlOn, translatIOn npple lammahon, gramfall lammatlOn and sand-flow lammatlOn (Hunter 1977) Thin, parallel laminae (parallel bed laminatIOn) develop on the stoss of eohan dune when the wmd velOCity IS too high to generate npples Accretion on the stoss Side of dunes produces translatlOn (chmbmg) npple lammatlon When the angle of chmb exceeds the dip of the stoss face, wavy lammatlOn and mternal cross-lamlllatlOn may develop ThiS IS the supercntlcal stage When the angle of cllmb IS less than the slope of the stoss Side due to a slow rate (subcntlcal) of accretIOn, tabular lammatlOn, a few mllhmetres thick, may develop, and the lamlllnae may also show mverse gradmg GralnfalllammatlOns develop when flow separatlOn by strong Winds causes depOSitIOn of saltatmg grams below the dune crest These may show poorly defined mternal structures When the angle of repose at the dune crest IS exceeded, en masse avalanchlllg may produce hnguOld flow lobes (sandflow lammatlOn) on the lee face of small dunes and cover the pre-eXisting sedimentary structures (see Chapter 7 and Fig 7 18)

Structures Related to Wave Ripples The Simplest type of wave motion IS caused by movement of wmd on the free surface of water At the water-aIr mterface globules of water move m Circles of the same diameter as the wave At depth the diameters decrease exponentially Wave npples are generated on a non-coheSive sediment surface when the OSCillatory motion of the water globules IS strong enough to move the grams The wave npples thus produced are usually str31ght crested m plan Only m exceptional cases are they smuous or three-dlmenslOnal The crests of wave npples are sharp and the round troughs are often mdented These npples are symmetncal m longltudmal section except when the forward and backward speeds of water waves are unequal It may be difficult to dlstmgUlsh between wave and current-generated npples on a purely morphological baSIS Current npples generally have longer lengths than heights At a rzpple mdex (R I = npple length/npple height) exceeding 15 only current npples occur (Rellleck and Wunderhch 1968) Symmetry m the outward pattern of a current npple may be produced due to eroSIOnal processes also, but the true nature of a npple m such a case Will be mdlcated by the mternal stratificatIOns

114


A

B

Fig. 5.23: Longitudinal sections of (A) planar-tabular and (8) trough-lenticular crossstratifications. Trench wall sections in a modern point bar, the Usri River, India. Length ot scale is 30 em (trom Sengupta 1974). With permission of Geological. Mining & Metallurgical Society of India.

The type of oscillation ripple generated on a bed surface is a function of the orbital speed (Vm ) and time period (1) of oscillation. Density, mean grain-size and shape of the sedimentary particles, and density and viscosity of the fluid are some of the other factors controlling the nature of wave ripples. A sequence of oscillation ripples of varying shapes and dimensions may appear as the orbital speed and time period of oscillation wave change (see Fig. S.24A). The fIrst to generate on a non-cohesive sediment bed at low orbital speed are small, straight-crested ripples with gentle slopes. These have been termed 'rolling-grain ripples' . With increasing orbital speed these ripples change over to steeper-sided 'vortex ripples' in which lee-side vortices erode and carry sand each time the flow direction reverses. The vortex ripples are stable over a wide range of orbital speed and time period. At higher orbital velocity and at a small to moderate oscillation period the vortex ripples change over to flatter, post-vortex ripples. During each half of the flow cycle these post-vortex ripples show strongly unidirectional flow. With an increasing time period and at a larger duration of one-way flow, the post-vortex ripples are converted to reversing-crest n'ppJes. At this stage the grain transport in each direction levels off the stoss sides of the ripples. When the oscillation periods are very long, both post-vortex and reversing crest ripples chaI1ge over to flat beds (Harms, Southard and Walker 1982). The internal stratification of an oscillation ripple is controlled by the rates of migration and aggradation (Fig. 5.24B). When the former is high compared to the


115

latter, ripple troughs erode parallel to the already deposited laminae and the internal strata dip in one direction only. On the other hand, when the aggradation rate is much higher, minor shifts in crest positions cause the laminae to be dovetailed in a chevronlike pattern. At intermediate stages, when some migration takes place in an environment which allows rapid vertical build-up, the climb angle is steep and the entire ripple profile is preserved (Fig. S.24B). Box cores taken from shallow marine sand (Newton 1968) show complex internal structures within features produced under oscillatory conditions. A large number of laminae dip randomly within such structures.

1. 0.----.,.-------,--,----,---,-....,....-,......-,

'B' 0.8

FLATBED

....... '"

g

~

'E ~ 3,

:eo

S

.§ ~ ~

Postvortex npples :'.. : .

0.6

." ." ~~D"""" ::.::.::,//

,

0.4

"", ...

Hummocky Stratification? .... ' .... VORTEX RIPPLES ...... ...... ... ... ...

- - --

-

(Start as rolling-grain ripples)

0.2

.'. . .... '

2-D

::~

: .~

.. ...'

. ... . ',"

~

.'

-':

'

:: ~~~ii~~~~;~;~ 'ri~~~~ (~~;:~~~~;'~;':-

No Rippling

O.l '---'-2--"-----'-4--'--...... 6--'--I8'--'-~10

@

©

®

®

Time period (sec)

Overturned wave ripple

@

Rate of migration

Overturned Cross-bedding forests

Cross-bedding foresets in original position

Fig. 5.24: (A) Physical conditions governing generation of wave ripples of various types (R indicates the fields of reversing-crest ripples). (9) Internal structures of wave ripples as functions of rates of migration and aggradation. (C·O) Internal structures of wave ripple and cross-bedding as indicators of depositional top and bottom of beds. (E) Cross-bedding foresets in original position. (A and B after Harms et al. 1982). With permiSSion of SEPM (Society for Sedimentary Geology).


116

Wave ripples have been reported from various environments but they are most common in nearshore areas and in tidal flats. Symmetric and asymmetric wave ripples of small magnitude, often with tuning fork-like bifurcation in plan, are found on margins of water bodies not more than a few centimetres deep. Symmetrical wave ripples with unidirectional internal stratification have been reported from shallow-water, near shore areas. With sufficient sediment supply from suspension, these may develop into climbing ripples or ripple laminae in phase. Features described in sedimentologialliterature as hummocky cross-stratification have been assigned to oscillatory flows of moderate to long periods (5-10 seconds) and high orbital velocity (>0.5 m/sec) corresponding to the field of three-dimensional vortex repples (Harms et al. 1982). On surface hummocky cross-stratifications (HCS, also called hummocky megaripples) consist of three-dimensional, circular or elliptical mounds (hummocks) separated by swales of large magnitude (L = I - S m, H = lOs cm.). The mounds are generally a few centimeters in height and the distances between them may range from 2m to Sm. In cross-section these features show sets of low-angle, concave-up laminations and generally low-angle (<:: I 0°) curved laminae intersections (Fig. S.25). These stratifications can hardly be used for palaeocurrent analysis because they dip in different directions. The swales having concave-up internal structures, constitute swaley cross-stratification (SCS).

Laminations drape hummocky surface

Sharp base; in places directional sale marks Sandstones commonly interbedded with bioturbated mudstone Fig. 5.25: Hummocky cross·stratification (HCS) commonly found in sandstones interbedded with bioturbated mudstone (after Walker 1984b). With permission of the Geological Association of Canadp.

The mechanism of origin of these features, which could hardly be produced in the laboratory or observed in present-day depositional environments, was a mystery for a long time. Marine fauna and bioturbated mudstone associated 'With some hummocks suggest their marine origin. Inner-shelf storm surges at depths varying between fairweather and storm wave bases (3D-SOm) were generally believed to be responsible for the origin of HCS. More recent studies suggest that the hummocks and swales are the outcome of combinedflows resulting from superimposition of unidirectional flows on oscillatory currents. During storm surges sand is carried into deeper sea in temporary suspension by high velocity unidirectional currents. At the same time the deposits are remodeled


117

into hummocks and swales by OSCillatory currents which reach below the ocean surface. Reported originally from the Cretaceous shoreface deposits of the Rocky mountams, USA (Harms et al. 1975), hummocky cross stratifications have now been found in stratigraphic horizons of many different ages and locations (Dott and Bourgcols 1982) Humocky megaripples (L = 2-5 m) with gently inclined internal 1ammations were reported from the mner Atlantic shelf of North Amenca by Swift et al. (1983) Arrays of these scour pits and mounds were produced by intense storms. The structures were later modified by wmd-driven flows. The processes involved are Similar to those responsible for generation of megaripples m a tidal environment. Bedforms produced by combined flows were reported by Banerjee (1996) from a Jurassic formation in Alberta, Canada. The size distribution of these bedforms was the result of a complex interplay of wave and current parameters together with cltmatic cycles of different periodicities. Swaley cross-stratifications were also reported from the Kansapathar Formation of the Chattisgarh Basin, India (Datta et al. 1999). These were produced in medium to coarse sand by OSCillatory and combmed flows. The relative frequency of trough cross strata and SCS within thiS formatton provided clues to the relative proximity to shoreline.

Structures Related to Flat Beds Bedding Bedding is defined as a sequence of parallellayenng within a body of sedimentary rock wherein individual layers differ from the overlying and underlying ones 1ll phYSical properties such as colour, composition or texture. Beddings are classified accordmg to their thickness. Sedimentary rocks often display a tendency to split along bedding planes. The commonly used terminologies for beddmg and splittmg properties) are given in Table 5.4. Table 5.4: Classification of bed thickness and splitting properties (modified after McKee and Weir 1953 ; and Potter, Maynard and Pryor 1980) Termmology for ThIckness (mm)

12000 6000 500 100 50 10 05

Beddmg

Spllttmg property

Very thick-bedded Thick-bedded Thm-bedded Very thm-bedded Thickly lammated MedlUm·lamlnated Thmly lam mated Very thmly lammated

}

MaSSive Blocky Slabby Flaggy Platy Fisstle Papery

Subaqueous horizontal beddmg may be produced either dunng settlmg from suspension (without tractIOn) or dunng transportatlon of sedlments by runnmg water In the lower flow regime, plane beds may generate when the gram-slZe IS large but the flow velOCity is small The upper flow regIme plane beds are generated when the gral n


118

~I/( I.,

.,mall but the flow vc\oClty I'> high The upper flow regime plane beds may be Identlhed by parallel arrangement of elongate mIneral grams producmg partmg lmeatlOn (FIg 526) now

s=:-;;;::&

J~~~~~~~~==-=~

1 ----=---.::.= ----.:::.---=--====--~- ----- ---- -------

Fig 526 Current lineation and parting lineation produced In the upper flow regime (after Harms et a/1975) With permission of SEPM (Society for Sedimentary Geology)

GradatIOnal, finmg upward sequences are common features m suspension deposIts Each layer of sediment wlthm a graded bed contaIns an assortment of grams, both coarse and fine, but the mean SIze of the grams gradually decreases upward (Fig 5 27) In the laboratory graded beds could be generated by allowmg sedIment laden flows to deposIt their load from suspensIon under deceleratmg veloCIty condltlOn (Kuenen arid MlgllOnm 1950) Some of the graded beds m anCIent marIne and lacustnne sedIments CDuld have been produced by turbId flows which progressively deposIted finer grams of sediments as flow VelOCIty and carrymg capacity of the current steadtly decreased Graded gravel beds are produced when slow movmg unsorted debns flows (mud flows) depOSIt theIr sedIment loads Gradmg may also be produced on the foreset of sand-dunes as a result of sortmg dunng bed load transportatIOn As stoss faces of dunes erode, and the matenals transported to the dune crest avalanche down the lee slopes, a vertlcal gradmg IS produced due to accumulatIOn of coarser grams towards the bottom On rare occaSIons dIsperSIve pressure caused by gram colltsIOn may force larger partIcles towards the surface of the shp face leadmg to what 15 termed mversegradmg(Bagnold 1954, Joplmg 1965) Some thmly bedded layers of manne orIgm are charactenzed by basal shell lags and a crude gradmg of carbonate clasts These have been termed 'tempesztes' because of theIr lllferred ongm by storm surges The term IS also used for storm generated sand layers produclllg graded rhythmltes (Remeck and Smgh 1980)

Structures Preserved on Bed Surface Shrmkage cracks, ramdrop Impnnts, water-level (recessIon) marks, WrInkle marks, swash, and nll marks are some of the common features preserved on top of a bed surface Shrmkage cracks may form eIther at the sedIment-au mterface by a deslccatlOn process (desiccatIon cracks) or at the sedIment water mterface by a synaeresls process In plan deslccatlOn cracks may look polygonal or non polygonal (FIg 5 28) where as synaeresIs Clacks are generally spIndle shaped or SInUOUS In cross sectIOnal vIew either may be U or V shaped DeSIccatIon cracks origInate by algal or mud shnnkage (mud


119

crads) due to (ltmosphenc drying 1 he~e drc frequently lI~ed as eVIdence ot ~lIbaendl exposure Surfdcc or sub~tratal dc watenng of submerged mud causes synaere51s cracks Dewatenng may be caused by the plesence uf concentrated bnnc, which sucks water out of the mud layer (Plummer and Gost1l1 1981) Dlstmctlon betwcen deSlccdtlon and synaercsls cracks bC1l1g dIfficult, a crack should be taken to be diagnostIc of subaenal exposure only when It IS assoCIated wIth other shallow-water, penodlcally exposed features hke raindrops, foam ImpressIOns or al11mal tracks

B

A

-- - -

- -=------: =--.. - ---~

::.--: ::. • ..• 'l,A."II ... ........ •... ........ :.: .. ..•...... ...• •

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':.~'::.

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;."

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' ,.~

'\

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~

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.., ............ ............. ..•...• ...... ..• •.~ •.. :fIP: •.• ;e:t11

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,,;~""",:~'.'

'.'

" ~:.-.

I

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•

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D

r

E

F

Fig. 5.27: Various types of graded bedding (after Kuenen 1953) Deep water lutltes are shown In black

Imprints of Intermittent Sbaerial E~posure Ramdrop zmprmts are preserved as crrcular, small, crater hke depressIOns with raIsed nms Other features produced by mtermlttent subaenal exposure Include parallel markings on slopmg banks (water receSSIOn marks), and wrmkle marks (Runzelmarken In German) The latter IS produced by the action of wmd on a thm film of water covenng coheSive sedIments These are mInute npple-hke features, a few mtlhmetres In length and only a fraction of a mllhmetre ll1 thIckness

120


Fig . 5 .28: Mu d cracks produced by subae rial exposure of modern flood ·basin depos its in the Us ri River, India .

Swash marks are indicative of the fa rthest line of encroachment of waves on land. These marks consist of lobate ridges of fine·grained sand which are convex towards the land . Broken shells, wood fragments or similar light materials often fringe th e e dges of these marks. Dendritic, bifurcating erosional features called fill marks are produced by erosion when water drains out of soft sediment surface at the falling wa ter level. These are indicative of thin water cover (Fig. 5.2 9).

Fig . 5.29 : Rill marks and worm trails in th e Us ri Rive r pOint bar, I ndia.


121

Current Crescent: Obstacles like shells, pebbles or wood fragments leave crescentic marks on soft sediment surface. These marks, called current crescents, are produced by

continued excavation of sand on the upstream side of the obstacle due to generation of vortex. The eroded materials are deposited on the downstream side in the form of a flare (Sengupta, 1966, Johansson 1976, see Figs. 5.30, 5.31). The flare, preserved in geological record, may serve as an indicator of palaeocurrent because it always opens up towards the down current direction. During the process of generation of a current crescent continuous excavation of bed material on the upstream side may cause the obstacle to tilt. Repetition of this process may lead to imbrication, a term coined to define overlapping shingling effect, generally of a series of flat pebbles (Fig. 5.32). Under suitable condition the scour width in current crescent preserved in geological record may serve ,as an indicator of flow velocity responsible for generation of the scour (Sengupta et at. 2005).

Downstream side

( Current direction

Upstream side

Fig. 5.30: Flow lines associated with a current crescent, plan view (after Sengupta 1966). With permission of SEPM (Society for Sedimentary Geology).

Structures Preserved on Undersurface of Beds

Scour marks of various shapes and sizes are produced when a current erodes an unconsolidated sediment surface. These are best preserved as moulds on the undersurface of overlying beds. Of these, the most common are the flute marks which occur as bulbous bodies, either singly or in rows, aligned parallel to the current direction (Fig. 5.33). Flutes flare and shoal out in the downcurrent direction to eventually merge to the sediment surface. Hence they may be used as indica tor of palaeocurrent direction. Flute marks may range in size from few millimetres to a few decimetres. The most important prerequisite for 'generation of a flute is a vortex caused by a turbulent flow. Experiments have-shown that flow generation accompanied by a strongly eddying current rotating in a nearly horizontal axis, transverse to the flow, is essential for generation of flutes (Allen 1973, 1975, see Fig. 5.34). Flute marks might generate at any defect on the bed surface but an obstacle, of the type necessary for production of a current crescent, is not essential in case of a flute.

a

122


Fig. 5.31: Current crescents around pebble barriers in a recent stream bed. Most of the pebbles are covered by leaves and other plant materials. The arrow indicates direction of flow (from Sengupta 1966). With permission of SEPM (Society for Sedimentary Geology). Downstream s' de I

_~.

Upstream side

___ .

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Stage-l

A

Stage-2

Stage-3

B

__

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Imbricate Pebbles Fig. 5.32: Longitudinal cross-sections across Fig . 5.30 showing stages of development of current crescents and consequent increase of plunge of the barrier pebble leading to imbrication (based on Sengupta 1966).


123

Fig. 5 .33: Isolated flute marks on undersurface of fine grained sandstone (Photo: cou rtesy of S.S . Das) .

(A)

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:

Fig. 5.34: Flow lines around flute mark . (A) Longitudinal profile showing separation of stream line . (8) Plan view showing skinfrict ion lines on surface of the flute and on the surrounding f lat bed (based on Allen 1968a) .

Tool Marks: Tool marks are produced on sediment surface by either stationary or moving bodies like a shell, a pebble, or a piece of wood. The tool mayor may not be preserved in geological record. The more important of these marks are grooves, chevrons, prods


124

and bounce marks These are bnefly descnbed below after Dzulynsla and Walton (1965), RICCI Lucchl (1970 cIted by ReIneck and Smgh 1980) Groove marks are long, lmear depresslOns produced by objects rolled or dragged on soft sedIment surface (FIg 5 35A) Groove marks are often preserved on the undersurface of flysch sedIments as sole marks. Chevrons are contInUOUS V-shaped marks (Vs pointing down-current) arranged in straight lines (FIg. 5 .35B). Tool 'dancmg'momentarily over cohesive sedIment surface, before advanCIng forward, produce these marks Prod marks are asymmetncal elongate V-shaped depressions produced by objects hIttmg a sedIment surface momentanly (FIg. 5.35C) Objects approachmg the sediment surface at a low angle produce bounce marks Symmetncal depressIons, tapenng both In the up-and down-current directions, are produced m the process 'The current soon lifts up the tool (Fig 5 35D)

~

<<.<<..<<<<< «« (A) Gloove malk

(C) Prod mark

(B) Chevron Mark (111 section)

Chevron mark (Ill plan)

(D) Bounce mark

Fig. 5.35: Tool marks of different types produced by bodies moving on soft sediment surface The movement pattern IS shown by arrows In each case. (A. B. C based on Dzulynskl and Walton 1965. with permission of ElseVier 0 - after RIccI Lucchl 1970 In Reineck and Singh 1980).

DEfORMED BEDDINGS Slumpmg, shdmg, unequalloadmg, and water escape are some of the inorganic agencies responsIble for distortion of layers of sedIments. Such deformations occur at the time of, or soon after deposition of the sediments but prior to their consohdation mto sedImentary rocks. Hence these structures are designated as penecontemporaneous deformation structures ('pene' means nearly) The process of mgestion of unconsolidated sedIments by orgamc agencies leadmg to their deformatlOn is called bioturbation. Penecontemporaneous structures have been grouped mto the followmg types by Reineck and Singh (1982) load structures, ball-and pIllow structures, convolute bedding, waterescape structures, dIsh structures and slump structures Smkmg of sand layers into unconsolIdated, hydroplastic mud may produce sand lobes varymg m SIze from a few mllhmetres to a few declmetres. The underlying mud


125

may be pushed upward in the form of tongues. At the sediment surface they may appear as bulges or knobby bodies. Casts of these load-generated features (load casts) may be preserved on the undersurface of beds as sale marks. When the sand layer is broken up into ellipsoidal, pillow-shaped masses and the underlymg clay is pushed upwards into the sand in the form of tongues, the feature is designated as ball-andpillow. Kuenen (1958) generated such features in the laboratory by subjecting a layer of sand overlying unconsolidated clay to vibrations. The sand layer in this experiment broke into discontinuous, kidney-shaped bodies, with the intermediate space occupied by clay (Fig. 5.36). Vertical movement of this type could have been responsible for most of the ball-and-pillow structures observed in nature. In some cases combined vertical and horizontal displacements due to slumping could have also been the cause.

,',::.;::::::.", ,':':.:; Sand

Mud

Fig. 5.36: Results of experiments on generation of pseudonodules by vibrating a layer of unconsolidated clay overlain by thin sheets of sand (based on Kuenen 1958).

Convolute Beddings Convolute beddings are characterized by folds with sharp, irregular crests (often resembling flames) and broad, round troughs (Fig. 5.37). They are restncted to single

126


sedimentation units, indicating that they are synsedimentary in origin. Sedimentary features of this type have been reported by many authors, both from ancient and modern sediments (see for example, Ghosh and Lahiri 1983).

Fig. 5.37: Deformed sedimentary structures. (A) Contorted beds in (?) sandstone, Chattisgarh Bas in, India. (Photo: courtesy of Sarbani Patranabis Deb). (B) Convolute laminations, Talchir Formation, West Bokaro Basin, India. (Photo: courtesy of H.N. Bhat1acharyay).

Convolute laminations occurring at regular intervals were interpreted by Kuenen to have generated out of sand ripples whose crests were sucked up by water currents. The sucking was inferred to have been caused by differential pressure at the crests and troughs of ripples. Differential loading due to deposition of sand from above (McKee and


127

Goldberg, 1969), vertical expulsion of fluid leading to increase in pore pressure (Allen 1977), upward movement related to vertical expulsion of water or gas and vortex motion of turbulent cells (Chakrabarti 1977), are some of the other interpretations offered for these structures. The contortions running across planes of stratification (named 'intetpenetrative convolutions' by Allen 1977) look similar to the box-shaped forms produced at boundaries of liquids of varying densities when heated from below. This also led to the suggestion that some interpenetrative convolutions are generated by transfer of material by processes similar to convection current. Some researchers have also assigned convolutions to deposition of layers of sand over water-saturated mud or silt in the initial stage, followed by downward movement of sand and a compensatory upward flow of silt or clay (see Pettijohn 1975). Such movements, it has been suggested, could be caused by a sudden shock, as in an earthquake. In reality, however, evidence of downward movement is rarely found in natural convolutions. The process seems to involve only unidirectional upward movement of silt and clay caused by squeezing out of material from below. When sand and mud are deposited together at a high rate, the sediment layers may be disturbed by escape of water unless the water can seep out of the system without disturbing the grains. The disturbances produced by water escape are of two kinds: (/) liquefaction caused by temporary suspension of sediment followed by settling consequent to collapse of the framework and (il) fluidisation involving lifting of the grains by the drag exerted by the fluid. Four types of water escape structures may be produced by these processes (Lowe 1975): 1) Hydroplastic or liquefaction layers produced by internal re-arrangement of sediment layers. Not much physical movement is involved in this process. Some of the convolute beddings, deformed cross-bedding and pillar structures come under this group. 2) Clastic sills and dykes produced by intrusion of hydroplastic, liquefied or fluidized sediments into the adjacent bodies of sediments. 3) Various types of deformed bedding or cross-bedding produced by downslope movement of sediment layers. 4) Dish structures produced by particle rearrangement. These are generated when sediment water slurries are forced into a nearly horizontal passage until they reach points of vertical escape. Dish structures occur parallel to the sandy bedding planes as concave-upward laminae a few centimetres thick.

Slump Structures Slump structures are caused by gravity flows of unconsolidated sediment layers. These may range from small-scale intraformational folds to large masses of slumps. Overturned or recumbent folds with associated thrust like structures are common within such slumps. Such· features have been reported from oversteepened point bars, gullies of intertidal flats, glacio-lacustrine and fluvio-glacial sediments. In the latter case an overriding mass of ice may cause the sediments to slump. Subaqueous cross-beddings may also be folded with or without overturning. A combination of folding and faulting is common in eolian dune sediments. Avalanching of masses of cohesive sediments on the slip face_ of eolian sand dunes may also lead to slumping. In coastal dunes deformation is aided by the cohesive nature of sand. Graben-like features, a few centimetres in width, may be produced by slump-related gravity faults.

6 STRUCTURES OF CHEMICAL AND BIOLOGICAL ORIGIN

Not all primary structures are of mechanical origin. Sedimentary structures may be produced by chemical and biological actions also. Segregation and local concentration of minerals within host rocks, leading to generation of concretions, nodules and the like are caused by chemical action. Features like stylolites are produced by pressure solution. Stromatolites and trace fossils are indicative of biological action. STRUCTURES OF CHEMICAL ORIGIN Concretions and Nodules Concretions are large spherical bodies whereas nodules are smaller. Both are produced by segregation and local concentration of a minor constituent within a host rock, for example, by accumulation of lime carbonate within shale and sandstone, silica within a carbonate rock, iron sulphide, phosphorous or barium within black shale. Manganese compounds may also lead to the formation of concretions. Segregation of the concretion forming mineral generally takes place around a nucleus, but its presence i~ not essential. Concretionary bodies are classified according to their composition, morphological characters or by the time of their formation. Compositionally, they maybe calcareous, siliceous, ferruginous or phosphatic. Morphologically, they are generally spherical, with concentric or radial internal structure but coarsely crystalline concretions with radial arrangement of acicular crystals are also known. These are named spheru!it~s or rosettes. Spherical concretionary bodies are common in sandstones. Isotropic permeability of this host rock allows free flow of the minor constituent in all directions. Where permeability is unidirectional, as in shales, the concretions are generally flat and elongated. According to their time of formation the concretionary bodies are classified as syngenetic, penecontemporaneous or epigenetic. Manganese nodules on deep-sea floor are good examples of syngenetic concretions. Penecontemporaneous concretions form before compaction of the enclosing sediment. Epigenetic concretions generate after consolidation of the host rock. Following is a composition-wise description of some of the common concretionary structures.

Carbonate Concretions The commonest calcareous concretions are caliche and calcrete. Caliche are thin, calcareous bodies occurring as crusts on soils which experience dry and wet climates 128

Structures of Chemical and Biological Origin

129

alternately Evaporation of groundwater drawn to the surface dunng the dry season allows preCIpitatIOn of dissolved carbonate Cahche may also occur on loess deposits of aeolme ongIn CalIche may show crude concentnc nngs, but the pnmary sedimentary structures are often oblItered In areas of low ramfall the carbonate dissolved m ground water may be mltlally precIpItated m the SOIl as encrustatIOn on roots of plants These encrustatIOns, called rhlzocretlOns, develop mto small calcareous nodules or glaebules, eventually coalescmg to form calcretes Calcretes are composed of mterlockmg crystals of low-magneSIUm calcIte and are much harder than calIche They may have well developed macro and mIcro fabrIc The cement of calcrete IS constItuted of sparry or aCIcular calcite grams Calcretes occur m vadose zone as dlstmctlve soIl honzons (pedogenic calcrete), wlthm or below the capillary nse zone The latter are called phreatic or groundwater calcretes The term calcrete IS used for "whole family of near surface, secondary carbonate accumulatIOns, and prefixes such as pedogemc, vadose or-phreatIC should be applIed when appropnate" (Tucker et al 1990, p 344) Well developed calcrete layers may display domal structures at the surface due to expanSIOn of calclUm carbonate These are named tepee Vanants of calcretes are known by dIfferent names ConcretIonary carbonate bodIes wlthm marly host rocks are called cornstone Sphencal bodIes, 1 cm to 10 m m dIameter, are called kugelsandstem In Germany These are produced by local preCIpItatIOn of carbonates around nuclei In sandstones and shales DISC shaped calcareous concretIOns occurnng wIthm SIlt layers m varve sequences are known as zmra stones m Scandmavla

Siliceous Concretions Chart and geode are the two common vaneties of SIlIceous concretIons Chart nodules are common m lImestones They are less frequent m sidente and shale They vary 1ll SIze from dISCS, a few centImetres m dIameter, to elongate bodies several centImetres long TheIr composItIOn vanes from monocrystalhne quartz to chalcedony They often show concentnc mternal features - a dark, black mtenor, surrounded by hghter coloured nngs Nodular charts and flInts wlthm lImestones are possIbly of epIgenetIc ongm Some of them mIght be the result of dIrect preCIpItation of masses of slhca gel on the ocean floor Geodes are sphencal or subsphencal bodIes of chalcedomc slhca dlsplaymg central caVIties The cavItIes are filled, commonly by proJectmg crystals of quartz, and less commonly by crystals of aragomte, chalcopynte or sphalente. Presumably, the outer shell IS produced by syngenetIc depositIOn of collOIdal masses of hydrated sIlica ThIS IS followed by solutIOn of the central portIOn of the geode and subsequent preClpltatlon wlthm the cavIty An alternatIve view assumes a calcareous concretion to be the precursor of a geode Yet another vIew ascnbes the ongm of cavIty to expansIOn due to generation of mternal osmotIc pressure wlthm a layer of semI permeable ge1atmous SIlIca membrane Shnnkage cracks follow dehydratIOn allowmg passage of mmeral beanng water mto the cavIty and depOSItIOn of a drusy hnmg over the pnmary chalcedony Ferruginous Nodules and Concretions Local segregatIOn of OXIdes and hydroxIdes of tron wlthm host rocks may produce ferrugmous nodules and concretIOns These may also be produced by partial

13 0


re place ment of limest ne pebbles by iron bearing minerals, or by oxidation of sideritic concre tio ns or claystone pebbles. Partially filled hollows (' box structures') may develop whe n jointed s iderite beds are oxidized to limonite. A variety of wired structures may resu lt when joints and bedding planes of sandstones are ftlled up by segregated iron oxide minerals. Nodules and aggregates of pyrite and marcasite are also known. Brown to black nodules and layers of hematite (fenicrete) may develop on top of iron-rich host rocks in a manner similar to the generation of calcretes on carbonate-rich soils.

Nodules of Phosp hates and S ulphates Phosphatic nodules have been reported not only from phosphate-rich deposits but also from limestones and chalk. They occur on present day sea floor also. Phosphatic nodules are black or brown with dens e, hard, shiny surface. Phosphatic spherulites, several centimetres in diameter, have been reported from the Cretaceous Montana Shale of Colorado. T hese are possibly the products of recrystaHiza tion of homogeneous phosphat ic concretions (Pettijohn, 1975, p. 476).

Products of Pressure S olution : Stylolites Serrated str uctures, called stylolites (Druksuturm in German meaning sutures generated under pressu re) are often found in limestones and marbles (Fig. 6 .1). These are produced by removal of rock material by interstratal solution Circulating under pressure. Solubility o f m inerals li ke calc ite, gypsum, and even quartz increase when subjected to pressure b ut do lomites resist press ure solution. Hence stylOlites occur frequently in calcareous and gypsiferous rocks, but r are ly in dolomites. They are also found , although infrequen tly, in quartzose sandstones.

F ig. 6 .1: Photomicrograph of a sty lolite in ooliti c limestone (Presidency College, Kolka ta collection . Courtesy of P rabir Oasgupta).

Structures of ChemIcal and BIologIcal Origin

131

~mcc stylolJtc~ are thl cffecb of prc\<;urc solutIon tl lcrtalll amount of poro~lty and permeabJllty of the ho~t rod.l~ l~<;ent1al for thL 1r gencratlon ThIs cxpIall1<' theIr ab~ence m shale The matenals thelt go mto <,olutlOn at the ~tylo!Jtc '>Ulfacc eVLntually fill the avaJlable pore space of the host rock thereby reducmg Its porosity and permeablhty and hmdenng furthcr growth of stylolJte~ 1 hus porosIty and permeabilIty of the host rock 1l1crease further from the plane of stylolItizatIOn The msoluble residues are deposIted at the stylolIte surface as dark films The tooth margIns of stylolItes are often marked by slIckensIdes mdIcat1l1g movement of one part of the rock agamst another A gross reductIon of the bulk volume of (he sedIment, to the extent of even 25 30% IS common In stylolItes The amount of reductIOn can be worked out from lateral dIsplacement of the vems transectmg the stylo!Jte surface Stylohtes transect mtergranular cement 1Tl blOT11lcntes and blOspaltes Clearly therefore, stylolItizatIOn IS a po~t-cementatlon phenomenon The stages leadmg to generatIOn of a stylol!te, envIsaged as early as 1926 by Stockdale, are as follows (a) ProductIOn of an undulatory surface along the part1l1g zone of a dIfferentIally soluble hmestone ThIS IS followed by (b) ll1creasc of pre')sure, apparently due to Increase of overburden The latter causes deepen1l1g of the ll1terpenetl atmg parts, pressure beIng greatest at the crests and troughs of the undulatIOns and least on the slopmg SIdes As a result the rocks on eIther SIde of the undulatIOn dIssolve, causmg further ll1crease of pressure More modern views presume the proces'> of stylolItIzatIOn to begm m sediments whIch are loosely cemented The process of ~tylolItI7atlOn, by releasmg calcIUm carbonate, leads the process of cementatIOn to completIon Growth of a stylolIte stops when permeabIlIty 111 the adjacent rock falls to a level that retards free flow of Ions Stylolites vary 111 sIze from a few mtlhmetres to several centImetres The largest stylolite reported IS from the Carbol11ferous Limestone of Yorkshire, where the amplItude IS as big as one metre (Bathurst 1975) Mlcrostylolttes occurnng m the carbonate rocks of the Late Precambnan BhIma Group of Karnataka, South IndIa, were classlfied on theIr geometncal aspects by Mal ur and Nagendra (1988)

Occurrence and Implications of Structures of Chemical Origin Innumerable reports on structures of chemical ong1l1 eXIst m geological Itterature Perhaps the most spectacular of these are the very large sIze calcareous concretIons m the Cretaceous Dakota Sandstone of Kansas The dIameter of some of these concretions go up to 9 m (PettIJohn, 1975) Calcretes have been reported, among other places, from the Old Red Sandstone Formation of Wales (Allen, 1965), and the Devoman Catskill FormatIOn of Pennsylvama In the Arran Corns tone Formation of Scotland carbonate concretIOns form complete beds, beddmg-concordant sheets, or beddmg-dlscordant nodules and rods These mIcntlc concretIOns formed close enough to the surface to be Incorporated, after erOSIOn and redeposltIon, as clasts mto the overlYlllg bed" Shnnkage played an Important role in subsequent fractunng and reworkmg of the mlcnte (Tandon and Fnend, 1989) Calcretes mlght be useful 111 palaeo-environmental InterpretatIOns In the Pennsylvaman shales, anCIent manne, freshwater and brackish enVIronments could be dlstmgUIshed by I3CI 12C ratio of sidente concretIOns (Weber, et a! 1964) CalCIC vertisols are a kInd of calciC so:l, dlsplaYlllg ~trongly vertIcal structures due to repeated deSIccation of expandable clay~ The~e reqUIre strongly seasonal conditIOn

132


for their depOSition whereas tropllal peats need contmUIty of preCIpitation Systematic alteration of coal and other paleosols (sec p 189) with calcretes and calCIC vertisols ill cyclothems of the Sydney basm Nova Scotia Canada, therefore, was Interpreted to Imply strong vanatlOn m seasonality In at least onc of these cyclothems, cabc paleosols formed on an mterfluve cut through a manne strata, suggestmg that a more seasonal and probably dner clImate prevailed dUrIng sea-level lowstand (Tandon and Glblmg 1994) Occurrence of calcrete IS particularly slgmficant because of ItS palaeocl1mattc Impitcatlon, the essential conditIOn for their generatIOn bemg a rainY season alternatmg With and or nearly and penods of hIgh evaporatIOn The calcretes found m the Late Tnasslc Malen Formatton of South India were mferred to be of slmuar ongm (Robmson, 1964) Later studies mdicated that these pelOids of mtraformatlOnal on gIn were denved by reworkmg of inCIpient pedogeniC calIche profiles which developed In Malen floodplaInS, abandoned channels and levees Other hnes of eVidence ltke the predommance of smectltes, poor floral content paucity of evapontes wlthm the Malen pomt to low seasonal ramfallm a semi-and environment (Sarkar, 1988) Tandon and Narayan (1981) reported the occurrence of authlgemc carbonates of three different vanetles In the contmental Slwaltk Group of India These are (1) calcrete conglomerate (2) 'case hardened conglomerate, and (3) cornstone The calcrete conglomerate COnsiStS of plSolttes which are concentncally layered carbonate particles formed from carbonate-nch spnng waters emergmg on to the gravelly substrate of dry, abandoned channels The 'case hardened conglomerate' With sparry calCIte cement has hmestone as ItS pnnclpal clast component It has resulted from cementatIOn of boulder conglomerate through contmued dissolutIOn and repreclpltatIOn of calCite by meteonc water Comstones are nodular calcretes These are a~soClated With thinly bedded sandstones and red shales of levee and overbank ongm respectively ThiS hthofaCles IS the result of concentratIOn of carbonate through capillary actlOn aSSOCiated With pedogemc actiVity The OOids m cornstones are mlcntlc A combmatlOn of clImatIC conditIOns, carbonate avallablhty, and sediment starved conditIOn mduced by volcamsm IS Inferred to have resulted to a stack of as many as fourteen calcrete profiles 1TI the dInosaur beanng mtertrappean regolIth of Central IndIa (Tandon, et al 1998) Of these the calcretes havmg a dlstmctive sheet-type of morphology formed m SOli vadose zone Promment shnnkage cracking IS mferred to have favoured rapId formation of calcrete and aSSOCIated rhlzocretlOn Calcretes have been used successfully m sequence stratigraphIc studies also Nodular and assOCiated groundwater calcretes m the coal beanng cyclothems of the Sydney baSIn of Australta mark the boundary between the underlYIng marIne and overlYIng allUVial depOSits They are mferred to represent lowstand surfaces (see chapter 9 for explanatIOn of the sequence stratigraphIc terms) MicrofaCies of calcretes show eVIdence of replacement, displacement and shnnkage, suggestmg that they were formed In relatively and condItlons ThIck allUVIal sediments WIth red calCIC vertISols overlYIng the calcretes are mferred to have formed WIthIn the transgreSSIve systems tract under condItIOns of abundant sediment supply The presence of calcareous palaeosols on lowstand surfaces suggests that the lowstands were times of relative chmatlc andlty (Tandon and Glblmg, 1997)


133

BIOGENI C SEDIMENTARY STRUCTURES Biogenic scdimentat y structures d rc or the ro ll owing three lypes (Frey, IlJ73).

1. Biostratification structures. This group includes stromatolites, byssal mats, biogen ic graded bedding and related features. 2. Bioturbation struct ures. This group includes tracks, trails, grazing patterns , burrows, and other disruptive structures. 3. Biodeposition structures. This group includes fecal pellets, fecal castings , pseudotraces, coprolites and the like. Stromatolites, d iscussed below, are the most important biostratification structures .

Stromatolites Lamina ted s tru ctures, p roduced by trapping, b inding and! or precipitation of fi negr ained , calcareous detritu s, by unicellular, filamentous organisms are called strom atolites . T he more common stromatolites are the ones related to algal growth , but stromatolites are not fossil algae. Algal stromatolites occur as laminated, mo undlike, columner or no n-colu mner m assses (F ig. 6.2 A & B) .

Fig. 6.2 (A): Plan view of hemispheroidal stromatolites ('domed algal mats' or Cryptozoons) in Late Cambrian Hoyt Limestone exposed on a glaciated surface at Lester Park, USA . The stromatol ite s are the products of intertidal environment.

Some features of inorganic origin like calcretes and geyserites resemble stromatolites in ou tward morphology but their internal structures are different. Calcretes, for example, display radial, granular or blocky interior instead of laminations. Geyserites are opaline silica deposited non-biogenically near hot springs and geysers. In external morphology some geyserites may resemble algal stromatolites, but unlike stromatolites they show banded internal lamination and at times, even micro -cross-Iamination. Moreover, unlike stromatolites they occur only around locales o f water discharge.

134


Fig. 6.2 (8): Columnar stromatolites in vertical section in the Neoproterozoic Chattisgarh basin, India. (Photo: courtesy of Sarbani Patranabis Deb) .

Stromatolites are found in limestones of all ages ranRing from the Precambrian to Recent. They were !nost abundant during the Riphean when diversity of columnar stromatolites was also the highest (see Fig. 6 .6 A & B). Originally thought to be restricted to marine environments, stromatolites are now believed to form in any freestanding body of water allowing growth of microorganisms. A high rate of sedimentation is essential for preservation of stromatolitic structures but too high a rate of sedimentation retards algal growth. The rate of growth of the stromatolite binding organisms must also be higher than the rate at which these organisms are consumed by other biota like gastropods. In fact , decline of stromatolites during the Phanerozoic is generally attributed to the advent of grazing gastropods feeding on the algae. High salinity of restricted pools keeps the gastropods away allowing better growth of blue-green algae. The rate of algal growth is also high in the upper intertidal and supratidal areas where grazing invertebrates do not survive prolonged exposure. Tidal channels are also congenial to growth of algal mats. Stromatolitic structures, both ancient and modern, have been studied extensively because of their importance in sedimentological and stratigraphic studies. A summary of the voluminous publications on stromalites appears in Walter (1976). Following is a brief review of the major ideas.

Morphology and Classification of Stromatolites Stromatolites are organo-sedimentary structures, not fossil organisms. Although their existence depends on the sediment binding property of algae, stromatolites themselves


135

lack the internal anatomical features of algae. The Linnean scheme of classification of algae cannot, therefore, be used, without reservation, for grouping stroma tolites. The stromatolitic forms built by filamentous algae today are divided into two g roups: cryptozoon and collenia. The former are laterally linked, vertically stakced, column ar hemispheroids attached to a surface. The latter are either discrete or laterally linked spheroidal or hemispheroidal bodies composed of concavo-convex laminae. Both the forms are similar to those found in geological record . The unattached, subspherical forms are called oncolithi. Hemispherical stromatolites lacking internal laminations are called thrombolites (Aitken, 1967). Three of the major schemes of classification of stromatolites based on external morphology are discussed below. Logan, Rezak and Ginsburg (1964) proposed a scheme of classification for stromatolites based purely on geometric criteria. Three basic combinations of hemispheroidal and spheroidal stromatolitic structures were proposed in this scheme. 1. Laterally linked hemispheroids (LLH) or linked domes. These are of two types. (a) Close lateral linkage (LLH-C), the space between structures being less than the diameter of the structures. (b) Spaced lateral linkage (LLH -S) , the space between structures in this case is greater than the diameter of the structures. The scale of lateral linkage may vary enormously, from microundulations, a few millimetres in height, to large-scale laterally linked domes a few metres across. The latter are commonly associated with preexisting irregularities (Fig. 6.3 A). 2. Discrete, vertically stacked hemispheroids (SH) of two types: (a) those having a constant basal radius (SH-C), and (b) those having variable basal radius (SH-V). Their scale may vary from a few centimetres to metres across the base. The SH structures develop out of algal cappings over preexisting irregularities like mud cracks or old algal structures (Fig. 6.3 B) .

(A) Fig. 6 . 3: Stromatolities occurring as (A) laterally linked hemispheroids, and (8 ) discrete, vertically stacked hemispheroids . (after Logan et at. 1964). With permiSSion of The University of Chicago Press .

136


3. Discrete spheroids (SS), ranging in size from fraction of a millimetre to about five centimetres or more. Three modes of occurrence are observed within thes'e forms. (a) Mode I-inverted stacked hemispheroids. (b) Mode R-randomly stacked hemispheroids. (c) Mode C-concentrically arranged spheroids. Instead of coating a surface the algal mat in this case coats a snell or a lithic fragment lying on the bottom. The size may vary from fraction of a millimetre to about five centimetres or more in diameter. (Fig. 6.4)

SS-R

SS-I

SS-C Fig. 6.4: Stromatolites occurring as discrete spherOids. SS-I: inverted stacked hemispheroids, SS-R: randomly stacked hemispheroids, SS-C: concentrically arranged spheroids. (After Logan et al. 1964). With permission of The University of Chicago Press.

Stromatolite colonies found in nature today contain complex combinations of the above m~ntioned types. Logan et al. (1964) used the following arbitrary scheme for describing these combinations of macro and micro structures. (a) Combinations of SH macrotype and LLH microstructure (SH/LLH) laminae. (b) LLH type passing to SH type, both on macro scale (LLH --* SH) (c) Alternating structural arrangements of LLB and SH types (LLH --* SH --* LLH). (d) Passage of LLH to SH on macroscale with LLH microstructure (LLH '--* SH)/

LLH). (e) Compound stromatolites having combinations of various LLH, SH, and SS arrangements (LLH-C -t SH-V '--* LLH-S ~ LLH-C/LLH-C). . In another scheme of classification proposed by Raaben (1969) columnar stromatolites were divided into four main supergroups based on their mode of ramification (diversification) and shape of the columns. 1. Conophytonida (non-ramified, with cylindroidal columns) 2. Kussiellida (passively ramifying, cylindroidal columns) 3. Tungussida (actively ramifying, columns widening upward, cup-like, with divergent axes)

137


4 Gymnosolemda (actIvely ramifyIng, pseudocylmdrOldal columns, axe" runnmg roughly parallel) Conophytomda columns are circular or oval m transverse sectlOn and conSIst of parallel stems of vaned SIze They form lens-like or stratIform blOherms, SOm or more m thIckness The supergroup KUssIClhda IS charactenzed by subcylmdrOldal, frequently elongated columns, which are Irregular In transverse sectlOn In the upper portIOn these are usually subdiVIded mto several mInor columns runmng parallel to one another Mlcrostrata are dome shaped, varymg In shape and height Marginal parts protrude Into the surrounding rocks Cup-lIke columns, mostly Irregular, charactenze the supergroup Tungusslda Diameters of these columns rapidly Increase upward Structures actively ramify Unlike Kusslelhda the ramified bush expands upward Axes of the columns are dIvergent Gymnosolemda are actively ramIfymg stromatolites WIth columns noughly parallel and pseudocyhndncal When well developed they have a narrow base and WIde top Hoffman (1976) proposed a morphologIcal claSSificatIOn of stromatolites based on hiS study of the modern stromatolItes In the Shark Bay of Western Australia These stromatolItes compare well With the ProterozoIc stromatolItes He recogmzed three major mat types colloform, smooth, and pustular Four other types (gelatinous, tufted, blIster and film) produce only stratiform sheets An mternal dIgItate fabnc of multlconvex lammatIOns produce a coarse fenestrate Within the colloform mats A fine fenestrate of convex laminatIOns occurs withm smooth stromatolitic mats The pustular mats have Irregular fenestrate fabnc Without lammatlons (Fig 6 5)

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Hoffman noted that the algal mat types do not control the gross morphology of stromatolItes For example, smooth mats may produce structures rangmg from stratiform sheets to high-relIef columns On the other hand, b1olog1cally dlstmct mat types may produce sIm1lar morphologies The phYSical processes of enVIronment ultimately control the d1stnbutlOn of mat types and also the stromatolIte morphology Thls aspect 1S discussed under environment


138

Stromatolites in Sedimentological and Stratigraphic Studies StratIgraphic ClaSSifIcatIon and CorrelatIon Stromatohtes have been studIed extenSIvely because of theIr use m stratIgraphy and sedImentology They were particularly useful In the claSSIficatIon and correlation of the otherwIse unfossIhferous Precambnan rocks (see Raaben, 1969 for a reVIeW of these studIes) On the basIs of the vanous assemblage zones of stromatohtes the ProteroZoIc was dlVlded mto pre-RIphean (2700-1650 MY), and RIphean The Rtphean agam was classIfied mto Lower (1650-1350 MY), MIddle (1350-950 MY), Upper (950-670 MY), and VendIan (670-570 MY) ThIS subdIvlSlon of the RIphean has been corroborated by radlOmetnc age data (see FIg 66) In the former USSR stromatohte zonatlOn was used for stratigraphIc correlatIon of extenSIve areas, and the VendIan was preClSely estabhshed WorldWIde apphcabIhty of stratIgraphic correlation on the basls of stromatohtes has also been confirmed. In the western part of the Taoudenm Basm, Mauntama, West Afnca, four stratigraphlc umts of Late ProterozOlc age were dIstmgUlshed and followed eastward as far as Algena Of these, the Atar or el Mrelt1 Group mcludes most of the stromatolIte beanng carbonate beds Two other groups also contam stromatolItes, and one group IS devOId of them (Bertrand-Sarfatl and Trompette 1976)

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139

Environmental Interpretation Association of flat pebble conglomerate, oohtes and cracked hl"lestoncs with stromatolites mdlcates shallow water ongm Cnnkmg of stromatoittic structures suggest drying, possIbly III very shallow mtertidal environment Upward conveXIty of stromatolitIc structures may serve as a clue to the deposltlonal top of a sequence Morphology of stromatohtic forms can be used as mdlcators of deposItional environment. For example, LLH-type stromatolttes (Collema) are found only m protected intertidal mudflats where wave actIon IS not strong. The SH -Type stroma tolItes (Cryptozoons) occur m exposed intertidal mudflats with strong wave action because scounng action of waves prevents growth of algal mats between stromatolites. SS-type structures (Oncolites) are charactenstic of low Illtertidal areas which are exposed to waves and agitated shallow water. The arrangement of lamlllae wlthm these structures provides clues to the frequency of movement Concentncally arranged spherOIds are the result of frequent motion whereas randomly stacked hemispheroids are the result ofless frequent motion (Logan, et al., 1964). Hoffman's studies m the Shark Bay enabled him to relate stromatohte morphology to the physical processes operatmg m dIfferent envIronments HIS major findmgs, summanzed below, proVIde clues to IllterpretatlOn of anCIent enVIronments 1. Stratiform crypt algal sheets occur where wave and tidal scour are weak 2. Discrete columnar structures occur in areas of strong wave and tidal scour. Their relief IS proportional to the intenSIty of wave action. 3. Elongation of SImple stromatolites IS parallel to the direction of wave propagatIOn and tidal scour, that is, generally perpendIcular to the shorelme 4 Elongate stromatolttes occur 1ll rows parallel to the shorelme 5. Stromatolites tend to ttlt seaward mto the oncommg waves. The pustular mats produce hIgh rehef cIrcular columns at headlands, medium to low rehef ridge-and-rill structures III bights, and flat pustular mats m protected embayments (FIg. 6.7). DimenslOns of these stromatolites also vary wlth environment At rocky headlands, irregular columns of coloform mats, upto 1.0 m in heIght occur These have discrete bladed branches with flattened tops. In the bIghts (a bend m a coast forming an open bay) stromatolites are more prolate and low (-O.Sm) m rehef On the subhttoral floor of tidal pond in embayment stromatolites with dIgitate columnar mternal structures occur beneath OVOId patches «O.Sm III dIameter). The smooth mat stromatohtic structures are large (-0 8m) at rocky headlands. Near bIghts this rehef IS reduced (Hoffman, 1976).

Stromatolite Occurrences in India One of the early reports m India of stromatolItes of ProterozoIc age was by Auden (1933). Interregional and mtercontinental correlation m the Lower Himalayan carbonate formations and Vmdhyans with the help of stromatohtes was attempted by Valdlya (1969), who used the term collenia for these stromatohtes RIphean stromatohtes were reported from the Jammu Limestones by Raha (1972) Geochronology of the Jammu Limestones, and also the stratigraphy of the Lesser Himalayas based on stromatohtic occurrences were discussed by Raha and Sastry (1973,1976). and by Raha, eta! (1978)


140

Chaudhun (1970) reported Precambnan stromatolites from the Pranhlta-Godavari valley Rudra and Mauhk (1987) reported stromatolttes of three morphological types domal stromatolttes, oncohtes, and cryptalgal lamInatIOns, from freshwater Lower JuraSSIC Kota hmestone of the Pranhlta-Godavan Valley Existence of these stromatolites withm the lake margIn sedIments, and also III deeper water was taken to be an mdlcatlOn of repeated submergence and emergence of the hme-mud In the Kota Lake Bight

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Trace Fossils Some orgamsms reSIdIng on sediment layers leave clues to theIr behaVIour m the form of traces traIls, tracks, burrows and borings These traces, preserved In rock record, are called trace fOSSIls or IchnoJosszls (lebensspuren III German meamng life traces) They are qUIte dIStlllct from body fossils Trace fossils are common 1ll sandstones and shales, but these have been reported from carbonate rocks also.

Classification oj Trace Fossils One of the pnmary methods of claSSIfication of trace fossils IS based on the positIOn of theIr occurrence With respect to the mterface of sandstone and shale layers (Seilacher 1964) Traces preserved wholly wlthm the bed are called 'full rehefs' and those occurnng


141

at the mterface of two layers of rocks are called 'semuehefs' Semirehefs agam, may occur either at the upper or at the lower surface of a rock layer. Those occurnng at the upper surface of a sandy bed are termed 'epueltef.' When a trace occurs on the lower surface (sole) of a rock layer it is called 'hyporelief.' Epire1ief and hypo relief may be positive or negative depending on whether they bulge up or down respectively (Fig. 6.8).

HyporeIJef (Hyplchma)

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Fig. 6.8: Classification of trace fossils based on the mode of occurrence (Based on Sellacher 1964, Lindholm, 1987. Terms within parentheses are after Martlnsson 1970).

In a similar scheme of stratonomic classification Martinsson (1970) named the traces preserved wholly wlthm a bed as endichnia I exichnta (burrows). Those on the top surface of a bed as eplchnia, and those on the bottom surface as hypichnia. The positIve reltefs were named 'ndges' and negative reliefs as 'grooves.' Various morphological and behavioural c1asslficatlOns of trace fossils have been attempted from time to time. The following classification, proposed by Seilacher in 1953, IS based on the inferred behaviour (hfe activity or habitat) of the animals concerned (see Frey and Sellacher 1980 for details). Name

Cubichma DOffilchma Fodimchnia Pasclchma Repichma Fugichma*

Habit

Restlllg Dwelling Feedmg Grazmg Crawlmg Crawlmg

(*Added to the hstby Simpson 1975).

Mode of occurrmce Traces of orgamsms restmg on soft sedIments. Burrows and borings as dwelhng places Marks of orgamsms searchmg for food. Marks of orgamsms exploitmg nutrIents Tracks and trails of ammals moving. Escape traces of ammals buned suddenly

142


Animals that temporarily dig into the substrate to create isolated shallow depressions make resting traces. Rusophycus is a modern example of such an animal. Dwelling traces are made by suspension feeders and scavengers which provide relatively permanent homes. These traces are usually cylindrical. Skolithos is an example of such an animal. Grooves and furrows made by deposit feeders like Rhizocorallium may exhibit complex behavioural pattern and make radial and V-shaped patterns in search of food. Mobile deposit feeders like Zoophycos make grooves and furrows that make most economical exploitation of nutrients in the sediments. Animals like Cruziana, which travel from one place to another, make crawling trackways and trails. Escape structures (Fugichnia) are made in response to substrate erosion or aggradation.

Use of Trace Fossils In Stratigraphy and Sedimentology Trace fossils are characterized by long time range but narrow facies range. These characters together with the fact that trace fossils show no secondary displacement and often occur abundantly in rocks, which have only a few body fossils, make them invaluable for stratigraphic and sedimentological investigation. When preserved as surfacial features trace fossils help determining the depositional top of a sequence. Under suitable conditions trace fossils also provide clues to the physical and chemical conditions prevailing at the time of sedimentation. Following are some of the examples.

Depositional Facies and Palaeo bathymetry The animals responsible for' generation of particular types of trace fossils are sensitive to environmental and bathymetric conditions. Following this clue Seilacher proposed that assemblages of trace fossils might be used for interpretation of depositional facies (,ichnofacies') and palaeobathymetry (Seilacher 1964, 1967). Following is a summary of Seilacher's major fIndings (Fig. 6.9). 1. In marine beach and intertidal zones, which are areas of rough water erosion and sedimentation, organisms like Skolithos and Arenicolites build deeply penetrating vertical tubes for self-protection. Both dwelling and feeding burrows dominate this zone. 2. Shallow shelves have Cruziana facies. 3. Relatively deep, tranquil water is characterized by Zoophycos facies. Zoophycos in general forms 3·D spiral structures. They branch out to build horizontal burrows. Such structures help them to collect food particles, which settle down from shallower levels and become part of the substrate. 4. In still deeper parts and within pelagic mud layers between turbidities, organisms like Helminthoida and Nereites splay out to collect the food trickling down from the water column above. As an objection to this apparently idealized interpretation of palaeobathymetry, it was pointed out that ichno-assemblages are controlled, not only by water depth, but also by other physical parameters like the quality of light, salinity and oxygen content

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144

of water. Direction of water current may also play an important role in controlling orientations of resting and grazing organisms (Ekdale 1988).

Ichnofossil as Indicator of Energy Leuel Some organisms churn-up the sediments, thereby disturbing the layers. The phenomenon is called bioturbation. Excessive bioturbation suggests vigorous organic activity together with a slow rate of sedimentation. Conversely, total absence of bioturbation indicates lack of organic activity possibly because the prevailing condition was anoxic. An environment devoid of nutrients will also have the same effect. Vigorous mechanical agitation, as in a beach foreshore, might also retard organic growth.

Ichnofossil

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Index of Toxicity

Burrowing activity is promoted in well-oxygenated environment. Increase of toxicity due to lack of oxygen or emission of H 2S may cause suppression of organic activity. Burrowing activity itself aerates freshly deposited sediments. Of all the animal species the O1)e producing the trace Planolite is least tolerant of low oxygen level. Thalassinoides is somewhat more tolerant, followed by Zoophycos and then Chondrites (Savrda and Bottjer 1986).

lchnofossil as Indicator of Strength of Substrate Mechanical strength and stability of the substrate can be inferred from the depth and condition of footprints of vertebrates, provided the weight of the maker (say, a bird or a dinosaur) of the print can be estimated. Corrugated vertical burrows and flattened horizontal tubes indicate that the sediments underwent post-depositional compaction.

Ichnofossil as Indicator of Rate of Sedimentation Seilacher (1962) interpreted absence of burrows within a sequence of flysch as indicative of very quick, essentially instantaneous, sedimentation. In a sequence of laminated, undisturbed sand and bioturbated shale, the sand is interpreted to have deposited quickly, and the mud very slowly, allowing it to be exposed to burrowing activity over a long period of time (Pettijohn, 1975, p. 131). It should be remembered however, that bioturbation of mud over a sufficiently long time may also leave a homogeneous residual layer.

Trace Fossil Occurrences and Reports Mu.:h of the pioneering studies on trace fossils and similar biogenic sedimentary structures are due to the German scholars like R. Richter, W. Hantzchel, and A. Seilacher. These studies were followed ih the United States by R.W. Frey, S.G. Pemberton and others. In India Arya and Rao (1979) reported bioturbation structures from the Middle Proterozoic Narji Formation, Kurnool Group in Andhra Pradesh. (Fig. 6.10). These structures, occurring in the form of worm burrows, were tentatively classified as belonging to the Skolithos and Glossifungites assemblages.

145

Structures of Chemical and Biological Ongm

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Mauhk and Chaudhun (1983) reported three types of trace fossils from the Tnassic red beds of the Gondwana sequence of the Pranhita Godavan Valley These mclude (a) honzonta! burrows (b) lllchned passageways between succeSSIve gallenes of honzontal burrows (c) vertIcal to slIghtly lllc1med burrows The morphologIes of these burrows are enVIronment senSItIve The traces are vertIcal In the Shlfilllg condItIOns of the channel facIes but they are honzonta! m the stable flood pi am areas Burrows and bIOturbated sedIments were reported from the slhClc1astlc tIdal flat depoSIts of the of the Bhander Group of the Precambnan Vllldhyan Supergroup near MaIhar, Central IndIa (Chakrabartl, 1990) The burrows vary from large-dIameter, nearvertIcal stubby forms to mIcroSCOPIC thread-lIke features cuttmg across the sedImentary structures Intense bIOturbatIOn of the sedIments housmg the burrows suggest that the trace makers were 'depOSIt feeders' Das and Rao (1992) reported numerous bIoturbatIon structures from the MIddle ProteroZOIC Chamuna LImestone beionglllg to the Chattisgarh Group, IndIa These burrows of mIcroSCOPIC dimenSIOns were obtamed from fine gramed, pyntiferous lImestone, depOSIted m dysaerobIC lagoonal carbonate mud enVIronment Based on their shape, Internal architecture, and theIr relatIOnshIp to the host rock, these burrows are belIeved to belong to a number of Ichnogenera SkolIthos, Monocratenonl Rluzocoral1lUm, and Polybessurus Sarkar and Chaudhun ( 1992) reported several morphological vanetles of trace fossus from the MIddle and Late Tnassic flUVIal redbeds of the Pranhtta-Godavan Valley The dIstnbutlOn of burrows here was shown to be faCIes controlled SkolIthos, a type of vertIcal dwellmg burrow, occurs both In channel and floodplam deposl ts Taellldl urn,

146


the horoizontal strucutures representing deposit-feeding organisms, are restricted to catastrophically emplaced sand sheets in channels and proximal floodplains. Vertical escape burrows are confined to continually accreting parallel laminated sands of channel bars. Horizontal dwelling burrows are confined to smaller sandsheets of floodplain drainage systems. Burrows of wormlike animals were reported from the Mesoproterozoic Chorhat Sandstone within the Vindhyan Supergroup of Central India. These burrows, unlike other trace fossils of similar age, always follow a bedding plane, which is a few millimetres below the original surface. Hence these were interpreted as the work of microscopic, worm like "undermat miners" that excavated tunnels underneath microbial mats. Their presence within the Vindhyan rocks suggests that triploblastic animals existed more 'that a billion years ago (Seilacher, Bose and Pfluger, 1998).

7 SEDIMENTARY ENVIRONMENTS AND FACIES

INTRODUCTION: FACIES MODELLING The term facIes, denved from a Latm (also French) word, Imphes a facIal expresslOn charactenstIc of a partIcular condItIOn In modern sedImentology the term IS used as an expresSlOn for 'the total field aspect of the rocks themselves The key to the mterpretatlOn of faCIes IS to comb me observatlOns made on theIr spatIal relatIOns and mternal charactenstlcs (ltthology and sedImentary structures) , (MIddleton 1978a) The process of construction of a faCIes model for an enVlfonment mvolves IdentIficatIOn of the dIagnostic or common characters of the partIcular enVlfonment thIS IS done by studying a large number of modern and ancient SItuatIOns and then formulatmg general prmclples about that enVlfonment after ehmmatIOn of the local detalis The purpose IS to obtam 'a norm for companson, a gUIde for future observatIOn and a predictor for new geologIcal sltuatlOns' (Walker 1978, 1984a) With thIS end m View, conSIderable effort has been made durmg the last few decades to collect mformatlOn about the modern envIronments of sedImentatIOn The followmg pages summanze the present-day knowled~e of the more Important envlfonments, manne, non, manne and mIXed A genetIc explanatIon for an ancIent phenomenon should come by way of analogy WIth these modern envuonments

MARINE ENVIRONMENTS TradItIonally the ocean-floor IS dIVIded mto three zones accordmg to depth nentlc(200m and above), bathyal (200-2000m) and abyssal (below 2000m) The deep-sea trenches are located m the hadal zone at depths exceedmg 6000m The more modern schemes use ocean-floor gradIent as the pnnclpal cntenon for claSSificatIon (Fig 7.1) The subdiVISIOns thus obtamed are contznental shelf(average slope 0 1°), contmental slope (4°), contmental nse (0 05°-0 6°) and abyssaJplam (005°) The faCIes assoCIatIons m these areas are bnefly discussed below after Walker (1984 b & c)

Continental Shelf The contmental shelf IS that area of the sea-floor lymg between the shorelme and the shelf break The sediments here are eIther slhclclastlC or carbonate dommant The sedIment dlstnbutlOn m slhClc1astlc shelves IS controlled malllly by three types of currents meteorologIcal (storm) currents, tIdal currents and llltrudmg ocean currents. 147

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Sedimentary Environments and Facies

149

Sfllclclastic Sediments on Shelf Strom-dominated Shelf. About 80% of the continental shelves of the world today are frequented by storm waves. These waves agitate the ocean-floor above the storm wave base, 'liquefying' the sediments and generating suspension currents of high density (turbidity currents) in the process. Deposition from these suspension currents below the wave base takes place in the form of turbidities. Hummocky cross-stratifications are found below the storm wave base in ancient deposits. The features which dominate modern shelves are sharp-based, cross-bedded, linear sand ridges a few kilometres wide, several kilometres long and 3-12 m thick, running subparallel to the shorelines. Ancient counterparts of these sand waves are lacking. Tide-dominated Shelf. Seventeen per cent of the world's continental shelves today are dominated by tidal waves. In the North Sea, adjacent to the coasts of England and the Nethertands, tidal waves have generated large-scale subaqueous sand waves over extensIve areas ranging between 5000 and 15,000 km2 (Fig. 7.2). The crests of these sand waves, which are about 35 m high, lie below 10 to 15 m of water, with their steeper sides dipping at low angles. Smaller sets of cross-bedding, dipping up or down the master bedding surfaces, constitute the internal structures of these sand ridges. Similar features have been reported from the Atlantic coast of North America also. Shelves Dominated by Oceanic Currents. The continental shelf south-east of Africa, which is under he influence of a major oceanic current called the Agulthus Current, is an example of such a shelf. Extensive elongate strips of sand waves, measuring 20 km x 10 km x 17 m approximately, occur on this shelf at a depth of about 40 m below the Indian Ocean surface. These sand waves have been generated by reworking of the sediments already present on the shelf. No comparable ancient example is known.

Carbonate Sediments on Shelf Study of limestone facies in a large number of geologically ancient basins, followed by their interpretation, using Holocene sediments as keys, led Wilson (1975) \0 formulate a generalized model of carbonate sedimentation on shelf. He recognized (t) an inner, shallow, low energy zone (undathem or shelj), (it) an outer, deeper, low energy zone (fondothem or basin), and (iiz) a shallow, high-energy zope (clinothem or shelf margin) separating these two. Wilson-further classified the three zones into nine facies belts covering the whole gamut of carbonate sediments on shelf, ranging from deep marine basin to evaporites and sabkhas inland. Following is a brief account of these nine standard facies belts summarized from Wilson 1975 (Fig. 7.3).

Belt IA: This belt is divided into two groups: (a) geosynclinal troughs filled with rapidly deposited limestone turbidities. (b) slow sedimentation with intermittent debris flows ('leptogeosynclinal deposits'). Sediments in these two belts are as fol1{)ws. (a) 'Allodapic' limestones-basinal sequence of breccias, exotic boulders, lime sands derived from shelf and slope carbonates. Their thickness is often great. Sedimentary textures and structures of terrigenous flysch are common. (b) Leptogeosynclinal troughs do not contain extensive allodapic limestones. These are only intermittent sites of sediments from outside the basin. Sediment is chiefly pelagic, mostly siliceous sediment accumulated in water deep enough for extensive solution of calcium carbonate. The rock types include radiolarites, red


150

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blomlcnte and red nodular lImestone, hght-coloured pelaglc hme mudstone, dark coloured bas mal mlcnte, pelagic mlcropelO1ds, bIoclastic grainstones and packstones etc These rocks are generally cherty, the sIlIca bemg denved from solutIOn of the opalme SIlIca of orgamsms llke mlcroplanktons. The strata are generally thm bedded Crumplmgs and contortlOns due to channelmg down of breCCia are notIced SyngenetIc conglomerates and breCCIas formed by solutIon and growth of concretlOns at sea floor may be seen The bIota of geosynchnal strata IS chIefly pelagIC

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152

Introduction to SedImentology

Belt IB: CratonIC Basm Carbonates (Starved, Mostly EUXlnlC) (Fondothem) Location Deep mtracratomc and margmal cratomc (rnlogeosynclmal) basins removed from carbonate producmg shelf areas ContnbutlOn from land IS slIght, often wmd blown A starved and somewhat deep basm results Physico-chemical conditIOns generally eUXlmc, stagnant and reducmg, below oxygenatIOn level and below wave base Water depth thirty to several hundred metres Bottom water hypersalme and dense Rock type dark thm beds of lImestone With dark shale or SIlt and some thm bedded anhydnte Terngenous clastics ltke quartz Silt and shale are admIxed wIth carbonates Chert IS common Colour Dark brown or black Sedimentary structures Planar lammatlon, cross lammatlon, small scale rhythmic beddmg BlOta Nektomc pelagiC fauna abundant Graptohtes, planktomc b .valves, ammonItes, sponge spicules Belt 2: Shelf FaCies (Deep Undathem) PhYSICO chemical condition water depth 1O-100s metre deep Normal salInIty Good current circulatIOn Shelves are Wide Sedimentation - UnIform Rock types Fosslhferous hmestone mterbeded With marl Colour Grey, green, red, and brown

Sedimentary structure thm to medIUm, wavy to nodular beds Ball and flow structures Thoroughly burrowed Mud-mounds, pmnacle reefs Terngenous clashcs quartz Silt, siltstone and shale occur mterbeded With lImestone BIOta diverse shelly fauna Brachiopods, corals, cephalopods and echmoderms present Belt 3: Basm Margm or Deep Shelf Margm FaCies (Clmothem) LocatIOn toe of a carbonate producmg shelf, below wave base and barely at oxygen level Water depth as m Belt 2 or perhaps 200-300 m deep Rock type Fme gramed ltmestone, at places cherty Terngenous clastiCS rarely present Chert common Colour Dark to hght

Sedimentary structures lammated hme mudstones, rhythmic beds MaSSive unlammated hme mudstone Some graded beds Megaslump features Biota mIxture of older forms denved from the shelf, benthomc orgamsms hvmg on the slope and some pelagIC forms Belt 4: Foreslope faCIes of Carbonate Platform (Manne Talus, Clmothem)

LocatIOn above the lower ltmlt of oxygenated water Rock type and texture vanable types of lImestones, hme mud and sand, boundstone sedImentary breCCIa Lime SlIts and bioclastiC wackestone packstone Reworked materIal WIth locally derIved organIC debns, reef rudstone

Sedimentary Environments ~nd Facies

153

Colour dark to hght SedImentary structures mega slumps m thmly bedded strata Large exotIC blocks SyngenetIC slumps, pull-aparts, breccias, clastic mJectIon dykes Terngenous clastics shale, slit, fine sand dnfted from above mIX With carbonate or ftll caVItIes Biota mostly bIOclastIc debns from upslope and colOnIes of encrustmg orgamsms Belt 5: Orgamc Reef of Platform Margm

LocatIOn and physiral condItIOns three types of profiles are known (I) downslope accumulation of carbonate mud and orgamc detrttus (ll) ramps of knoll reefs With bIOclastIc sands (m) frame-constructed reef nms Rock type maSSIve lImestone and dolomIte m places conslstmg solely of orgamsms BlOclastlc debns Patches of orgamc boundstone WIth mterstlces filled With hme mudstone Gramstone and packstone In upslope accumulatIOns Colour hght SedImentary structures massive organIc framework Terngenous clastICS: essentially absent BIOta colomes of sesslle frame-buildmg organIsms may dommate Belt 6: Wmnowed Platform Edge Sands

LocatIOn and phYSical condItion sholas, beaches, offshore or tIdal bars, aeohne dune Islands Margmal sands range from well above sea level to 5 to 10m deep Clean sand IS wmnowed and depOSited by waves, tidal, or longshore currents EnVironment IS well oxygenated but not hospItable to manne hfe Rock type and texture calcareous or dolomItic l1me sand Rounded, well-sorted gramstones, often oohtiC or rounded blOclasts Colour hght SedImentary structures medmm to small scale festoon cross-beddmg Eollan dunes wIth large cross-beddmg Old soIl honzons and root casts Terngenous clastICS quartz sand and calcaremtes BIOta worn coquInas of benthOnIC ammals hvmg on reef and foreslope Large bIvalves and gastropods common Fragmented remaInS of large algae and some foramlmfera Belt 7: Open Manne Platform FaCies (Shallow Undathm1)

LocatIOn: m straIts, open lagoons, and bays behmd outer platform edge PhYSical condItion Shallow water, a few metres to tens of metres deep SalImty vanes from normal manne to somewhat higher CIrculation IS moderate Water conditIOn SUitable for growth of orgalllsms but stenohalme forms are excluded Rock types and textIlre vanable lImestone and lenses of land denved clastICS Gramstone to mudstone Lenses of !tme sands of shelly and angular fragments Beds of bioclastIC wackestones, bIOstromes Colour lIght and dark

154


Sedimentary structures: medium to pJaty bedded burrowing and pelleting of sediment

common. Ball and flow compaction structures, wavy bedding. Terrigenous clastics: when present, generally in well-segregated beds intercalated with limestones. Biota: mollusks, sponges, arthropods, foraminifera and aJgae common. Patch reefs present. Fine sediments occur trapped in marine grasses. Organisms (e.g. brachiopods, echinoderms, cephalopods) requiring normal marine salality are present but may be rarer than in open sea. Belt. 8: Facies of Restricted Circulation on Marine Platform Location: restricted lagoons behind or between barrier reefs, behind coastal spits or within atolls. Generally shallow, hypersaline water, both reducing and oxygenated conditions. Diverse intertidal environments. Rock types and texture: mostly lime mud with dolomite. Grainstones rare. Channels contain lithoc1astic grainy sediment, pelleted mudstones and wackestones. Colour: light. Sedimentary structures: laminated mudstone, fenestral (birdeye) fabric, algal stromatolite, graded bedding, dolomite and caliche crusts. Cross-bedding in channel sands. Terrigenous clastics: rare except wind-blown material in well-segregated layers. Biota: limited fauna and flora. Gastropods, algae, foraminifera (miliolids), ostracods abundant locally. Belt 9: Platform Evaporite Facies

Location: Supratidal and inland pond environment of restricted marine platform in evaporative c1imate-sabkhas*, salinas, salt flats. Sporadic marine flooding. Rock types: nodular and wavy anhydrite or gypsum interlaminated with dolomite. Commonly associated with redbeds. Colour: variable. Red, yellow, brown. Sedimentary structure: laminate, planar, mud cracks, stromatolites. Gypsum rosettes and selenite blades. Nodular and flaser structures. Caliche crusts. Terrigenous clastics: redbeds and windblown sediments. Biota: No indigenous fauna excepting blue-green algal stromatolites and brine shrimps.

* Sabkhas are depressions formed in extremely arid conditions (for example deserts) by removal of loose particles by wind. Sabkhas go down to the groundwater level or to the zone where concentration of salts drawn by capillary action takes place. In marginal marine conditions, in arid climate, the tidal flats subjected to periodic tidal influx may lead to hypersaline pools. Such conditions have been termed platform evporite facies by Wilson (1975). The typical deposits within these pools are laminated dolomite together with nodular anhydrite, gypsum and halite. Some wind-blown terrigenous materials may also be present. Burrowing organisms are absent due to hypersaline conditions. The most well known coastal sabkha today is III the coastal areas of the Persian Gulf.

155


Continental Slope The slope borderIng the contlllental shelves are marked by networks of submanne canyons and gullIes lNater-wet terngenous sedIments are carned m masse by gravIty mto the deep sea through these canyons The phenomenon IS called 'mass-transport' Three major types of mass-transport are recogmzed-rockfalls, rockshdes anfl sedImentary gravIty flows Rockfalls cause talus accumulatIon at the toe of slopes RockslIdes can be eIther translatIOnal (glIde) or rotatIOnal (slump) Several types of sedImentary gravIty flows, from debns or mud flows to turbIdIty current flows, are known In debns flows the clasts are supported by the coheSIve strength of the mud matnx The water-wet mud behaves hke a plastIC body In a turbIdIty current the clasts are supported by flUld turbulence and the whole mass behaves as a flUId ThIS also mcludes lIquefied flows m whIch the sedIment IS supported by upward dIsplacement of fluId (dzlatance) , and flUIdIzed flow In whIch the sedIment IS supported by upward mot10n of escapmg pore flU1d Intermediate between plastIC and flUId flows are gram flows m whIch the sedIment IS supported by dlspetSlve pressure (see FIg 7 4)

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Continental Rise and Deep-sea Fans Temgenous sedtments transported to the ocean floor through feeder channels and canyons splay out to produce deep-sea jans GravltatlOnal processes effect transportatIOn of these sedments These fans constitute the major part of the contmenta1 nse Of the many different flow processes through the feeder channels the followmg three are more tmportant

Debris Flows Debns flows are produced when a m1Xture of clay and water acqUires a densIty hIgher than water and transports large clasts of sand and even gravel The large-SIze graIns are


156

deposIted when the transporting medIUm looses Its densIty and becomes mcreasmgly flUId DepOSItIOn takes place en masse Carbonate Debris Flows The debrIS flows and turbIdites are not necessarily composed of sIhCJc matenals only Carbonate sedIments also contnbute to turbIdItes Corals and algal fragments, obvlOusly of shallow water ongIn, are sometimes transported to much deeper water by turbIdIty currents and depOSIted there Spectacular carbonate slump masses, called oltstoltths, have been reported from Nevada m USA and also from Italy Carbonate debns flows consIst of clasts of varymg SIzes embedded In hme mud Enormously thIck carbonate debns flow depOSIts have been reported from the Cambro OrdovlClan rocks of Newfoundland Carbonate turbIdItes are known In the Alps and Apennmes Carbonate turbIdIty gram flows and debns flows of recent ongm occur m the Bahamas m USA Grain Flows

Gram flows take place when InteractIOn between the grams tS strong enough to produce dIspersive pressure forces WhICh keep the grams apart Scattered grams floatmg m a sandy matnx are deposited out of thIS process Sole marks are common

Fluidised Flows FlUldtsed flows result when mtergranular pressure mamtams the grams In suspensIon Transportation occurs WIth Increase of mterstltlal flUId pressure DepOSItIon takes place when the dIfference between mterstItial pressure and hydrostatIc pressure decreases All the above mentlOned processes contrIbute substantially to turbIdIty current depOSIts The dIstances through whIch the dIfferent types of sedtment travel on the ocean floor depend on their gram-SIze and concentration (see FIg 7 5) The net volume of these mass flow depOSIts can be enOrmous - of the order of many CUbIC kIlometres GravitatIonal Processes

I FlUIdIzed flow Intergranular ascendIng flow

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Gram flow I nteractlOl1 between glaIllS

Debns flow Coheston of the matrlX

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En masse depOSitIOn

Fig 75: Mechanism of cohesion and depOSition of sediment In aquatic environment by gravitational processes (based on Middleton and Hampton 1976)

157


These can be transported with astonlshmg velOClty For example a maxImum veloclty of 20 m/sec was estImated for the turbidIty current produced by the Grand Bdnks Earthquake of 1929 (Dunbar and Rodgers 1957)

Depositional Sequence Within Deep-sea Fans Bouma Sequence The sequence of sedImentary structures produced wlthm a submanne fan by the depOSIts of mass flow IS as follows (see FIg 7 6) e pelItIC dIVISIOn d upper dlvIslOn of parallel lammatIOn C current npple lammatlOn b lower dIVISIon of parallellammatIOn Bottom a graded or massIVe beds

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ThIS sequence, Ideal1zed by Bouma (1962), has been named after hIm The complete Bouma sequence (Ta-e) IS rarely found In geologIcal record but even a partIally preserved Bouma sequence may help m the IdentificatlOn of an anClent fan depOSit

158

IntroductIOn to Sedimentology

Morphology oj Deep-sea Fans The followmg generalized facIes model for deep-sea fans was formulated by Muttl and RICCI-LucCI (1975) on the basIs of studIes by Normark on modern deep sea fans and by Muttl and assoCIates on the anCIent fans of the Apenmnc'i, Italy An IdealIzed fan, accordmg to thIS model, consists of the followmg components (FIg 77) AnCIent Fan

Modern Fan Normark

Lucchl (1975)

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(1978)

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BalIn Plam \\here Pll~LUt)

Fig. 7.7: A Simple classification of submarine fans proposed by Normark (1978), RlcclLucchl (1975) (modified after Howell and Normark 1982) With permisSion of American Association of Petroleum Geologists

An Inner (or upper) fan wIth one or more feeder channels Typical lithofaCIes conglomerates, coarse-gramed to pebbly sandstones Some remobiltzed deposits suggestmg mass slumpmg may be present Basal scounng and channelIng are common A mzddle fan consIstmg of a number of sha/low, branchmg channels- feedmg a depositIOnal lobe (the 'suprafan' of Normark 1978) TYPIcal Itthofacies basal gravels and mud ChIPS overlam by coarse to medIUm sandstones WIth mterbedded fine-gramed sandstone and mudstone The sandstones deposied by turbIdity currents are of ulliform thIckness for a long dIstance and exhIbIt a classIcal Bouma sequence An outer (lower)fan, which begms beyond the limIts of the dIstnbutary channels, contams tabular mudstones WIth mterbedded, graded, thlll sandstones The upper part of the Bouma sequence IS preserved III these deposIts Sole marks are common The deposItional lobes, WhICh are topographically smooth, grade laterally Into a basm plam The basm plain (lower fan of Normark 1978) COnsIstS essentIally of pelagIc and hemIpelagIC mudstones WIth a few graded sandstones The depOSits are transItional WIth the outer fan fnnge

159


Abyssal Plain Deposits The abyssal depths of the ocean are dark and cold Blue-grey to lead-coloUied mud transported from land to the ocean-bottom by suspensIOn currents. floor the bathyal zone of the ocean, but the abyssal zone IS covered wlth red-coloured mud ThIs red mud, havmg the chemical compositlOn of an average Igneous rock, IS beIteved to have been blown mto the deep sea from volcanIC vents located 10 the adjoIning land areas Prolonged oXldatlOn of fine-gramed terrestnal particles slowly setthng through oxygenated water columns might also be responSIble for the red colouratlOn In relatively deep water, upto a depth of about 3000 m, pteropod ooze, cocohths and foramlmfera predommate The carbonate preClpltatlOn In this zone IS essentially III the form of aragonIte (orthorhombIc CaCO J ) Between 3000 and 5000 m globlgenna ooze, conslstmg mostly of low-magnesian calcIte (hexagonal CaC0 3), dommates the ocean floor In the deeper parts of the ocean, where the water IS cold, pH IS low and pressure IS hIgh, carbonate remaInS mostly In solutlOn Increase of partial pressure of CO2 (pCO z) due to dIssolved CO 2 at great depth also allows ocean water to hold carbonate m solutIOn The depOSIts of the abyssal ocean floor contaIn only about 20% carbonate, whereas the depOSIts In the shallower parts of the ocean contam 90% The depth above whIch calCIte-nch sedIments accumulate IS called the calCite compensatIOn depth (CCD) Below thIS depth only calcIte-free sedIments are found because the rate of calcIte supply IS balanced by calCite dISsolutIOn at CCD Deepwater carbonate, wherever found, has been eIther transported from the shallower parts of the ocean by suboceanIC denSity currents, or dropped onto the ocean floor from the pelagiC zone where floaters and SWImmers abound Cocohth ooze, formed by the latter process, IS an Important constItuent of the fine-gramed lImestone called chalk

()

40"

\

40 G

120" Fig. 7.8: ApproXimate direction of Increase In the depth of carbonate compensation (CCO) In the major oceans of the world

Calcite compensatIOn depth vanes from ocean to ocean but Its average depth IS about 4500 m In the PaCIfic Ocean CCD occurs at a relatIvely shallow depth (42004500 m) In most of the North AtlantIC and In some parts of the South AtlantIC also (between latItudes 40 0 Nand 40° S), It IS at 5000 m, or stIll deeper In the equatonal

160


Pacific CCD is about 5000 m. Increase in the amount of biological productivity leading to abundant supply of calcareous material accounts for this increase in CCD near the equator. In the Indian Ocean CeD is at about 5000 m near the equator but its depth decreases to about 2000 m at 60° latitude (Kennett 1982, Tucker et al. 1990). (Fig. 7.8). The biochemical precipitates below the carbonate compensation depth are of siliceous nature. Local concentrations of ferromanganese concretions ('manganese nodules') are found in some pockets of the deep-sea floor. These are ascribed to exhalations from suboceanic volcanic or hydrothermal vents but the precise mode of their origin is not known. Marine Sediments in Stratigraphic Record

The typical facies associations found in marine shelf, continental slope and continental rise are briefly described below.

Shelf Sediments in Stratigraphic Record Sedimentation on continental shelves can take place under three conditions: transgressive, prograding and aggradational (Galloway and Hobday 1983). Transgressive sequence may be either tide-dominated, or storm-dominated. Both of these produce fining-upward sequences. The transgressive sequences start with lag gravels at the bottom and end with bioturbated silts and muds on the top, passing successively through coarse and fme-grained, cross-bedded sands (Fig. 7.9A). The stormdominated transgressive sequences start with lag gravels at the bottom and end in silty and muddy sediments on the top, p-tssing successively through silts and muds (with lenticles of sand), multistorey sets of hummocky cross-stratification and thinly graded, parallel laminated sands (Fig. 7.9 C). The hummocky cross-stratifications are storm generated. The thinner sets are deposited as the depth of water increases due to transgression. Laminated mud, rich in organic carbon, glauconitic and phosphatic muds may be laid down on top of the ')uccession of a deep shelf deposit. Many of the outer shelf mudstones are phosphatic with a very high organic carbon content. Some of these have been correlated with the phases of worldwide eustatic transgressions by Vail et al. (1 (77). The prograding shelf sequences typically coarsen upward. Two prograding sequences, one dominated by storms and the other, a mixed energy shelf succession, have been identified. The storm-dominated prograding successin is almost the reverse of the stormdominated transgressive sequence WIg. 7.9 B). A progressive seaward fining of sediments, caused by the storm-induced sorting processes, is represented as a coarseningupward vertical sequence. The typical sequence produced in a mixed energy (storm and tidal) prograding shelf is represented in Fig. 7.9 D. Aggradational shelf sediments are generally fine grained. Silt and sand are deposited by traction currents. Mudstone is deposited when the rate of sediment supply is slow due to gradual rise in sea-level or alternatively, due to subsidence of the land area. Periodic transgression and progradation may cause sandy sediments to alternate with thick sequences of mud. In anci(!nt geological record mixed energy aggradational sandy shelf successions are represented by thickly cross-bedded quartz arenites.

161

Sedimentary Environments and Facies Lammalcd

U

U

cdrbon,'leou~

muds

U \I

V

U

Troughs

U

\I

II

U

U

Bioturbated

U

U SlitS and muds

U

U

U

U

U

DIspersed cross beds

Hummocky cross-stratificatIon

=--::: PdfdllelldmmatlOll

Large I ndmed ~urface~ with troughs

Reactivation Large ~scalc foresets

II

U

U

"

U

II U Graded silty U lam mae

U

U U

BlII rowed

U

©

U

U

/;-/// ",;-//'

U

u u ill

@

fhm bedded U II

U

~Bioturbated mud & silt

~ Cross-bedded sand

t§ Parallel laminated sand t:;:J

U U U

Lag gravel

Troughs Plana I cross bed,

Thm bedded Burrowed mterbar slltltonc '---"'~ Troughs ///// PlanH cross beds ~/;'~/~ ~I

Lammated carbonaceous muds

®

Lag Burrowed or bored subjacent UnIt

x,-

~hclf

muds

V

U

Legend

Storm-graded bed,

U U

U

®

Stonn graded beds

Hummocky cro~~ stratlfi(atlon

U

Lag

Burrov.ed glauconitic silty muds

U

U

v

13lOlurbated gld UCOOl!lC 'llty mudstone

II

Fig. 7.9: Idealised depositional sequences in (A) transgressive, tide-dominpted shelf, (8) prograding, storm-dominated shelf, (C) transgressive, storm-doml .....ted shelf and (0) prograding shelf dominated by storm and tidal actions (aftpr Galloway and Hobday 1983). With permission of Springer-Verlag, Heidelbery.

Continental Slope and Fan Deposits is Stratigraphic Record The deposits of the continental slope and talus include debris sheets, rudite sheets and sedimentary breccias of very large dimensions. The latter are called olistostromes (Greek: Olistomai-to slide, Stroma-layer). Derived from shallow water siliciclastic and carbonate deposits, the slope breccias may be of diverse composition, varying from well-bedded calcareous sands and calcarenites to rounded blocks of reefal lImestones resembling bioherms. Downslope they may grade into deep-water pelagic or hemipelagic sediments_ The pelagic sediments are open sea deposits containing calcareous ooze, skeletal remains of foraminifera, pteropods and coccolithophorids. The terrigenous component is small « 20%) in these sediments but very fine-grained pyroclastics (tephra) are common. Siliceous (radiolaria, diatoms) ooze and skeletal remains are common below the Carbonate Compensation Depth. Hemipelagic sediments are admixtures of fine-grained pelagic and coarser grained terrigenous components. Deep-sea fans occur on the ocean floor.

162

Introduction to Sedimentology 2

The largest submanne fan known today covers an area of about 3,000,000 km on the Bay of Bengal floor. About 2 billion tons of sediments are washed into this fan annually through the' Swatch oj No Ground' submarine canyon, to be distributed through a channel-levee system up to the northern Indian Ocean. The biggest of these channels is about 27 km wide and 180 m deep (Fig. 7.10).

Fig. 7.10: Sketch map showing extension of the Bengal and Indus Submarine Fans. Based on Map of the Indian Ocean Floor by National Geographic Society (1967).

A submarine fan migrates through space and time to produce a vertical sequence which is preserved in the geological record. A progressive migration from the proximal to the distal areas (from the inner to the outer fan) may be caused either by uplift of the source area, or by an increase in the .rate of sedimentation. Lowering of the sea-level will also have the same effect. In the vertical succession produced from this process the I coarse-grained inner fan deposits overlie the fmer -grained outer fan sediments (Fig. 7.11 A). When the rate of sediment influx decreases, or the sea-level is raised, a retrogradation of the submarine fan takes place. In the vertic'al sequence the coarser inner fan deposits are successively ovet'Min by the progressively finer middle fan and fan fringe sediments (see Fig. 7.11 B). Many examples of marine shelf deposits exist in geological literature. The Lower Palaeozic sections of the upper Mississipi valley and the Great Lakes of North America

163

Sedimentary EnvIronments and FacIes

are often cIted as tYPIcal examples In both the areas shelf sedIments were deposIted on the southern extensIOn of the CanadIan ShIeld when the conditIOns were very stable (Pettijohn 1975) The Jura QuartzIte of Scotland and the Gog QuartzIte (Lower Cambnan) of the Southern Rocky Mountams are anClent examples of tide-generated sand bodIes (Walker 1984b) ReefOldal complexes are recorded from all geological ages but coral reef buIlders evolved on a large scale from the SIlunan onwards The SIlurIan reefs of Gotland, Sweden, the Devontan reefs of Alberta, Canada, Carbomferous reef complexes of North England, the PermIan reefs of New MeXICO and Western Texas, Tnasslc reefs of the DolomIte Alps, Itlay, and the TertIary reefs of the MedIterranean and the PersIan Gulf are some of the most famous examples of carbonate reefs m the geologIcal record ..--.

...--t.-

f-i...:

".Jl

•

~:-r . . . .

1

Submanne Canyon

, ,. • .• •

Fan fnnge

.. . 1 • J }

.~

\

J

Inner fan

Mlddle fan

\ "i

MIddle fan

~ -,

L

.....:::= I

t

.

=: •

.-.. , ·ci..

} Fan frmge ...-

--

.;........:.

® b

. . . I.

Pebble-Coarse Sand

Submarme Canyon

.0 :'0'0

LEGEND

1

Inner fan

Fme Sand

j

Silt-Clay

Fig. 7.11: Examples of vertIcal sequences produced by migratIon of submarine fans (A) a progradational sequence, (8) a retrogradatlonal sequence (based on Howell and Normark 1982) With permission of American AssociatIon of Petroleum Geologists

In IndIa the shale-quartzlte-aremte sequence of the KaJrnur Formation tn Rajasthan IS beheved to have been deposited dunng the ProterozOIc by an epmc sea on a very gently dlppmg, storm-dommated contmental slope Many sedImentary structures, mc1udmg hummocky cross-stratIficatIOns, have been reported from these sediments (Bose, Chaudhun and Seth 1988) The Cretaceous Nlrnar Formation of M P, IndIa, was also mterpreted as the product of sedimentation on a storm·cum faIr weather dommated contmental shelf (Bose and Das 1986)

164


Extensive marine sediments have been reported from the Himalayas by many authors. In one of the more recent studies 'a complex jumble of volcanic and sedimentary rocks' occurring between Lamayuru Formation (Triassic to Cretaceous) in Ladak, and the Triassic flysch in southern Tibet, have been described as 'ophiolitic melange' (Thakur and Virdi 1979; Virdi 1986). The Lamayuru Formation in Ladak contams exotic masses of limestones which are believed to have accumulated at the toe of an ancient continental slope by slumping. Olistoliths and grain-flow deposits have also been reported here (Virdi 1986). NON-MARINE ENVIRONMENTS Glacial Environment Glaciers are masses of moving ice produced by recrystallization of precipitated snow. Two types of glaciers are recognized: valley glacier, and continental ice sheet (Figs. 7.12 A, B). On crossing the snow linea valley glacier starts losing its volume by melting and evaporation and eventually reaches a zone where the amount of wastage exceeds the amount of accumulation in the upper reaches. The annual loss of volume of a glacier is called ablation. As the snout of a glacier vanishes by meltmg, the glacier is said to recede. A moving glacier shatters the bedrock by exerting pressure on it. The shattered rock fragments, frozen at the base of the glacier, scratch and abrade the pavement below, producing striations. Gouging and scratching also produce 'chattennarks'. As a glacier slides downhill the pavement is sculptured to asymmetric mounds called roches moutonnee (French for 'rock sheep'). A series of irregularly spread mounds, called 'drumlins', is also produced by smoothening of a glaciated terrain. Such features form an Ubiquitous part of the landscape of North America and Europe, which were covered by extensive sheets of land ice during the Quaternary. The deposits ('drifts') produced out of glacial action are of two types: stratified and unstratified. Varves, outwash deposits and eskers are examples of stratified drifts. Unsorted mixtures of morainic gravel, sand and mud (designated as 'tiff in older literature) are called 'diamicts' _ Lithified equivalents of diamicts are 'diamictites' (meaning a thorough mixture of mud and gravel, from Greek: diamignymi - to mix thoroughly). Glacial facies are described here under two headings: continental and marine, following Eyles and Niall (1984).

Continental Glacial Facies These include grounded ice and also, glacio-fluvial, glacio-lacustrine and periglacial facies. Ground ice grows into ice sheets of continental proportions. Two types of facies associations are possible within ground ice sheets, depending on whether the ice margin is inactive, that is, frozen to the substrate, or actively sliding over the bed. In the case of an inactive margin, the frozen basal debris is draped by the diamicts of melt-out englacial debris. These may be resedimented to fill basins elsewhere. These basin fills are eventually exposed as 'hummocks' after melting of the surrounding ice cover. Typical vertical profiles of these hummocks show unstratified, in situ aggregates of meltwater diamicts at the bottom, overlain by graded and stratified diamicts which are the products of re-sedImentatIon


165

®

Fig. 7.12: Schematic representation of the deposits associated with (A) land ice sheet and (8) valley glacier.

When the glacier continues to slide over the bed, being continuously lubricated by the basal meltwater, faceted clasts result from intense abrasion (see Figs. 3.1 and 3.2), These dasts, with a strong preferential alignment to the direction of ice movement, are incorporated into the basal diamicts. They may be overlain by channel fills produced from subglacial stream drainage. Eskers, the deposits of the subglacial streams, may be of various types and forms, depending on the site of deposition and the nature of the conduit tbrough which the glacial stream flows out. In channels bordered by high walls, antidune structures may develop due to high·flow velocities, whereas within tunnels, parallel or cross-bedded structures dominate. These structures are preserved in vertically stacked cycles. Deltaic sedimentation at the mouth of the tunnel produces subaqueous fans: The proximal parts of these delta are marked by cobbles and pebbles. The grain-size decreases downstream (Banerjee and McDonald 1975). A glacio-fluvial facies is produ.ced from the deposits of the meltwater draining the outwash plains. These deposits, which may extend to hundreds of lGlometres, are called

1&6


sandar(plural for sandur, an Icelandic term). Glacial rivers which drain sandar are braided and their deposits are typically coarse grained. These deposits show a progressive decrease in grain-size downstream, ending, ultimately, in coastal fans. In a humid climate the swamps within these outwash plains may produce peat (or even co~l) by decomposition of the subarctic vegetation. In a dry climate fine silt may be blown out of the outwash fans and dropped in the downcurrent areas as extremely will-sorted deposits called loess. Diamicts Resedimented diamicts Glacio fluvial! lacustrine facies

In situ diamicts, Glaciotectonically deformed substrates Fig. 7.13: Section through a typical 'hummock' (based on Eyles and Miall 1984).

Lakes may be produced within the continents in the areas evacuated by ice sheets. Two types of glacio·lacustrine facies are identified: periglacial and proglacial. The former, not in direct contact with the ice margin, is fed by braided stream systems. Varves with a sandy or silty lithofacies deposited during the meltwater (summer) season, followed by 'winter clays' are common in periglacial lakes. Varves, traditionally believed to be seasonal deposits ('annual couplets'), have also been interpreted as graded units of a Bouma sequence deposited by turbidity currents within periglacial lakes (Lambert and Hsu 1979). The silt unit ('summer layer') of a varve may be graded by density underflows with pelagic materials incorporated from suspension (Banerjee 1973). A diamict lithofacies forms a minor constituent of the periglacial sequence. PerigIaciallakes, which are in direct contact with the ice sheets, are fed by meltwater conduits, subaqueous fans and by ice rafting. The accumulation of suspended sediments and stratified rafted debris (diamictites in these lakes) are overlain by cross-bedded, deltaic silty sand. Dropstones are common. The Talchir (Lower Permian) deposits in India may be cited as examples of continental glacial facies. The sequence at Dhakoi, for example, starts with a conglomeratic facies at the base and is followed upward successively by massive sandstone and a fine sand to shale facies (Fig. 7.14). The conglomerates were deposited as longitudinal bars within progIacial outwash plains during retreat of the Talchir ice


167

lobe Blocks of dIamictites enclosed wIthIn these conglomerates represent Ice-rafted morainIC matenals The overlYIng sandy facies, whIch IS often cross-bedded, represents channel sands and migratmg bars of braided streams The finer clastics were deposIted tn protected or abandoned parts of glacIal outwash plaIns A finmg upward cycle was produced by ShIftIng of the braided channel bars withm the glacIal outwash plams (Casshyap and Tewan 1982)

NW

SE

Fig. 7.14: A typical Talchir sequence exposed at Dhakol, India (afterCasshyap and Tewan 1982) With permission of SEPM (Society for Sedimentary Geology)

Marine Glacial Facies

Two dIfferent types of glaclo-manne sediments, proxImal and distal, are recogmzed. In the proxImal areas dense, sedlment·laden meltwater produces bottom currents and submarine fans. DtaIntcts WIth ice-rafted clasts are common depOSIts at the fan apex. Downstream, the depOSits are much finer, cross-bedded sand formIng the mlan fan body with mud deposIted from suspenSIOn Pebbly dIamIcts may be Incorporated Into this deposit by gravity flows. In the steeper channel banks hOrIzontally stratrlied and mversely graded gravels are deposIted by gravity flows A generalized model for manne glaCial faCIes was proposed by Edwards (1986), based on PleIstocene examples Four depOSItIOnal areas wlthm manne glaCIal faCIes were recognIzed Isostatic, submanne retreat, margInal and proglacial These depOSIts, Edward assumed, are controlled by the relatIve effects of rISe and fall In the sea-level due to eustatIC and Isostatic causes

168


Recognition of Ancient Glacial Environment Geologically young glaciated terrains may be recognized by the presence of such geomorphic features as roches moutonnees, eskers and drumlins. But these surfacial features are rarely preserved in older geological records. An association of striated pavements, chattermarks, varves and diamictites with typically pentagonal, flat-iron· shaped, faceted pebbles indicates glacial activity. Glacially transported quartz grains are characterized by conchoidal breakage patterns, semi-parallel steps, arc-shaped microchattermarks, parallel striations, imbricated breakage blocks and irregular indentations. These surface textures, identifiable under transmission electron microscope, are diagnostic of glacial transport (Krinsley and Donahue 1968). Tillite beds of Upper Carboniferous to Lower Permian age have been reported from all continents of the Southern Hemisphere. Parts of the Dwyka of South Africa, Tubarao of Brazil, Kamilario of Australia, and the Talchir of India may be mentioned as examples. The glacial origin of the Talchir Boulder Bed of India, proposed by pioneer workers of the Geological Survey of India, was later confIrmed by a host of geologists. At places such as Irai, central India, glaciated striae were incised on the underlying Precambrian limestone pavement by Lower Permian ice movement, allowing faithful preservation. The direction of plucking and chattermarks on the underlying limestone, elongate troughs and striae enabled determination of the direction of ice movement (towards NE and NNE) at Irai (Smith 1963). The Talchir glacio-lacustrine shales and siltstones in the Raniganj Coalfield in eastern India are punctuated by sandy beds with a graded, sole-marked base and laminated top. Slump structures, parallel current-ripple and convolute laminations, mark these laminated beds. Varve-like rhythmites containing current-generated structures such as current-ripple lamination are also found within these glacio-lacustrine sediments. All these are interpreted to have been produced by turbidity current activity. These density currents are envisaged to have been triggered within the outwash glacial lakes by slumps caused by oversteepening and overloading of an ancient delta front (Banerjee 1966). Eolian Environment Eolian activity is strong in an arid climate - in sand deserts located in rain-shadow zones within continents, and in glacial outwash plains. The climatic conditions in these areas do not permit growth of vegetation and wind blows unabated for long distances. Eolian activity in coastal dunes is independent of climate. Vegetation plays an important role in stabilizing these dunes. Disintegration and exfoliation of rocks in a desert environment is caused mainly by exposure to sunrays. Sediment transportation is essentially by suspension and saltation. The impact of thesaltating grains may cause the finer particles to creep over the surface. without losing contact with the ground (suiface creep). The impact of the sand grains in motion (deflation) is another phenomenon responsible for erosion in deserts. Bedforms of various dimensions are produced by the action of wind: ripples (wavelength 1..= 0.01-10 m), dunes (A = 10-500 m) and draas (A. = 0.5-5.0 krn). Each of these may be further subdivided into transverse and longitudinal elements leading


169

to a total of eight subgroups, all apparently independent (WIlson 1972). A more comprehensive scheme of classification of eolian bedforms is given In Table 7 1 (after McKee 1979). WIlson's draas are recogmzed in thIS scheme as the result of combination of two or more bedforms of the same type. Sand seas of still larger dimensions (-10,000 km 2) are named ergs. These are produced by redistnbution of sand initially deposited elsewhere. Table 7.1: Morphology and classification of eolian bedforms (after McKee 1979) Morphology

Name

Sheet-lIke Thm elongate stripS

Sheet sands Streaks

Circular to elliptical mound, dome-shaped

Dome

Crescent in plan Connected crescents

Barchan Barchanoid

Asymmetrical ndge

Transverse (reversing)

Symmetrical ndge Central peak with arms

Lmear (seif) Star (pyramidal)

U-shaped

Parabolic

ASSOCIatIon

Compand-two or more of the same type combmed by overlap or supenmposltlOn (Wilson's draa).

Complex-two different baSIC types occurrmg together, either supenmposed (Wilson's draa), or adjacent to each other

The external morphology and internal structures of the more important eolian bedforms are shown in Fig. 7.15 (A and B). With increasing complexity of wind regimes the dune form changes from simple transverse to complex star dunes and draas (see Fig. 7.16). In the sand roses shown in this diagram the arm length of a rose is proportional to the amount of sand that can be moved by the wind from a given direction towards the centre of the rose. (Stereonet plots of wind movement prepared from the sand roses will give a diametrically opposite picture). With a decrease in Resultant Drift Potential (RDP) and the RDP IDP ratio, the pattern of sand accumulation changes from lateral migration to vertical accretion. The Drift Potential (DP) is a measure of the total sand-moving capability of the wind expressed in vector units (v.u.) without regard to wind direction at the locality. RDP is a measure of the resultant or net sandmoving capability of wind at a locality in vector units (Fryberger 1979). Thus RDP / DP measures wind directional variability. By plotting RDP IDP against Equivalent Sand Thickness (EST), Wasson and Hyde (1983) could discriminate the four major dune types: linear, barchan, transverse and star (Fig. 7.17).

Eolian Facies Models Two different models, one applicable to transverse bedform migration and the other to longitudinal bedform migration, have been proposed (Brookfield 1984). In the transverse model, unidirectional wind flows, sand patches and barchan dunes are initiated WIth the onset of aridity. With increasing aridity, these change over to transverse dunes and


170

compound transverse draas. These forms decrease m size with decreasmg andity and sand supply. In the longitudmal model the inltial sand patches and barchans are followed subsequently by linear dunes, linear complex draas and star draas. These longltudmal patterns are rarely found m anClent eo han dunes. Slmple Dunes Longltudmal SectIons

Vlew

Barchanoid

-------......----. V

V

U

100 m

10-100 m

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A Plan A

Transverse SectIons

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Linear

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ca

100m

r=::J Gram fall and chmbmg trans1atent

1 ........ I AdhesIon structures

f;::; I Sand flow

1--- I Mud crocks

G=:J Coarse lag sands

Fig. 7.15: Schematic representation of ~tratlflcatlOns found In (A) simple and (8) complex eolian dunes of different types. The longitudinal sections are parallel to the resultant wind direction. the transverse sections are perpendicular to It (redrawn from Brookfield 1984) With permission of Geological ASSOCiation of Canada

171


®

COMPLEX AND COMPOUND DUNES

: , . . . . a

BarchanOld draa ~---Plan VIevv

Wmd 1001000 m

modes

~

-- :: 10-100 m

;-~

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Interdune

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10005000 m

m

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1001000 m

I ongItudmal (II to mIgr dtrectlOn)

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o o o

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I

n erdrd

:

Draa

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\

;. . . ... --- ---=-= B

Gram-fall and chmbmg translatent Sand flow Coarse lag sands

Fig. 7.15: (8)

Increasmg CompleXIty, Wmd Regunes and Dune Forms Transverse BarchanOid ndge dune

~

~

dun/?

if

~ _OIILs::::m~e~a:ariid~un~e~ ~

Star dune

-Zi1Vh

Lateral MigratIOn

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Vertical Accretion ----7

WIde UnImodal

Obtuse Blmodal

A

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North

WalVIS bay S Atnca

BahraIn ArabIa

Badanah. Arabia

Tlmlmoun. Algena

R D P AND R D P /D P RATIO DECREASES

Ghadames LIbya

)

Fig. 7.16: Relationship between eolian dune forms and Wind regimes, DP = Drift Potential, RDP Resultant Dnft Potential, RDP/DP IS a measure of lateral migration. The proportion of vertical accretion Increases WIth decrease of RDP and RDPI DP (after Ahlbrandt and Fryberger 1982) With permission of Amencan ASSOCiation of Petroleum Geologists

=


172

50 STAR

e

40

tl

30

-'"""'

TRANSVERSE

......., II I~

20

E-< r./) ~

10

LINEAR

BARCHAN 0.5

0

RDP/DP

1.0

x = Icy

Fig. 7.17: Plots of equivalent sand thickness (EST) vs. wind direction variability (RDPI DP) as a tool for discriminating different types of eolian dunes (after Wasson and Hyde 1983).

Vertical sections of eolian deposits worked out in the Permian Thornhill and Dumfries basins of Scotland show barchan dl,lnes marking the onset of a desert. These are overlain by large barchanoid draas corresponding to the main eolian phase when arid climate predominated. The overlying small dunes mark the dwindling eolian phase, which were superimposed by alluvial fans during semi-arid climate (Brookfield 1984).

Recognition oj Ancient Eolian Deposits Forms and magnitudes of dunes of eolian and subaqueous origin overlap to a large extent. These cannot be used as criteria for recognition of ancient eolian dunes, therefore. The presence of lag gravels on avalanche toes, convex forests, arcing either up or down, deformation and reactivation surfaces, and progressive flattening and thinning of crossbedding surfaces towards the top mark many of the present-day eolian deposits. Modern eolian dunes are also characterized by the absence of trough cross-beds. The upper parts of slip faces are generally steep (29°-34°). A combination of all these features might indicate eolian sedimentation although no single criterion can be taken to be diagnostic. Eolian sands, because of their selective mode of transportation, are better sorted than sand of any other environment. The presence of fmes in these sands skews the grain-size distribution positively. Rounding, pitting and frosting are common on the surface of quartz grains of eolian origin. These may be detected by scanning electron microscopy (Krinsley and Doornkamp 1973). A combination of sand-flow cross-lamination, climbing translation stratification and grain-fall lamination is indicative of the eQlian process (Hunter 1977; see discussions under sediment structures). Two types of eolian climbing ripple lamination are recognized; translatent laminae and ripple-form laminae. In the former only the bounding surfaces between the ripples can be seen but in the latter the forests are also visible. Grain-falI lamination is produced by deposition from suspension in the zone of flow separation. Sand-flow lamination is caused by downslope avalanching on the


173

lee face. Most of these laminations can be identified on the eroded surface of barchan dunes (Fig. 7.18). The degree of porosity may also help in the identification of eolian sediments. Sand-flow cross-strata are loosely packed and highly porous. Climbing translatent strata are tightly packed and less porous. The grain-fall laminae are intermediate between the two in porosity and grain packing (Hunter 1981).

o

2

4

6m I

PLAN VIEW

~

Climbing trans latent stratification

r.:-:-:-:l Grainfall1amination L:.;.:..:..:.:J r:=-=l Sandflow cross-

t::=..::J stratification -L

Strike & dip of cross-strata Boundaries between sets

A

Fig. 7.18: Plan and sectional views of a barchanoid dune showing different types of stratification and cross-stratification (modified after Hunter 1977). With permission of International Association of Sedimentologists.

The Permian Toroweap Sandstone of Arizona, the Mesozoic Navajo Sandstone of Utah and Arizona, the Aztec Sandstone of Nevada, and the Lower Bunter Sandstone of England are some of the well-studied eolian deposits. The Late Proterozoic Venkatpur Sandstone of southern India is interpreted to be of eolian origin on the basis of (t) grainflow strata, (it) inverse graded trans latent strata, (iit) adhesion laminae and (iv) grainfall strata. Nine facies, representing both eolian and related aqueous environments, have been recognized within what is believed to be a well-developed erg. easets of large-scale cross-strata are present within this sand sea which terminates at an alternate sequence of eolian and marine sediments. The horizontally stratified units within the Venkatpur sandstone have been variously interpreted to represent interdune, inland sabkha, sand sheet and coastal sand flat deposits (Chakraborty 1991).

174


Lacustrine Environment

Lakes o~cupy natural depressions surrounded on all sides by barriers or rims. These rims may be of tectonic or volcanic origin. Damming of river valleys, solution, glacial excavation and meteoric impact may also generate lakes. Organic activity is an important part of lake sedimentation. The upper, welloxygenated layer of water in lakes, where sunlight can penetrate, is replete with biological activity. The lower layers are cold and depleted of oxygen. The zone of rapid temperature variation between the two layers is called the thennocline (Fig. 7.19 A). A salinity layering may also develop in inland lakes connected to the sea. Ice capping on the lake surface in cold climate does not allow mixing of the upper and lower layers, but in spring, when the upper layer warms up to 4° C (the temperature at which water is heaviest), the lake 'turns o~er' due to settling of the heavy layer to the bottom. The phenomenon is repeated again in autumn. During periods of turnover, nutrients carried to the upper layer from the lake bottom enhance biological activity. Organisms settling down from the upper layers produce organic debris within the lake mud. Photosynthesis by phytoplanktons during the summer months removes much of the CO2 from the upper layer of water. The dissociation of bicarbonate that follows increases alkalinity, thereby aiding in precipitation of calcium carbonate. The presence of carbonate indicates a warm, alkaline condition at the lake-bottom, a cold, acidic condition being non-congenial to carbonate precipitation_ Together with fme clastics, the lake-bottom carbonate may produce marl. In the productive stage of a lake, floating microscopic algae lead to the development of a brown to black, lipid-rich organic sediment called gyttja. During the low-water stage, the zone of aquatic vegetation at the lake margin widens and a floating mat of sedges grows within the lake, covering the gyttja, reducing the lake area and making it smaller. A reducing and acidic condition converts this vegetable matter into peat (Fig. 7.19 C). A rise in water level, on the other hand, produces oolitic shoa~s and algal mats (Fig. 7.19 B). Unlike sea-water, complete evaporation of lake-water is possible, and a variety of salts such as sulphates, chlorides, borates and fluorides, normally not expected from evaporation of sea-water, precipitate within the lake sediments. Authigenic silicates such as feldspars and zeolites may also develop. Reduction of iron at the lake-bottom may cause precipitation of pyrite (FeS 2 ). Reducing and restricted basin conditions are needed for such precipitation. Reducing (Euxinlc) Basin

Hydrologically closed lakes, as also all other basins cut off from open sea by barriers or sills, enjoy reducing environment. The term euxinic, derived from the Latin name of the Black Sea (PontusEuxinus), implies 'cut-off or silled' condition. Being cut off from the Mediterranean by a barrier the Black Sea enjoys an euxinic condition (Fig. 7.20, X and Y). Other well-known areas enjoying euxinic conditions today include the Drams Fjord of Norway, Saanich Inlet of British Columbia and the Baltic Sea. To be euxinic a lake should be located in a climatic zone wherein seasonal 'turnover' of upwelling is not possible. The Black Sea is euxinic because it is located in a humid

I....

.::...-==---= -=-;-

o .

-..;

:-v

-=--.=-- -=--

...l.

2...' .-,

Fn sand, silt,

(mad)

'--y,:....!,:-'-..,!, CaIbonatr ':""r'"" --!,.. , ~ Ilch cia,

Peat

rootlet!>

.....

~_""(':..'Y.. clay .,.. _=i::.."::

.1

---

..J......!.- ..,

....:!

~ - ---.:---:-

lamIDaoon

DeSICCatlOn _ _ _ _ Clacies & parallel

:;~:z~:~~~~

---=...;;=r;r;...;;~

:::--::

-

StromatolItes

= =-===

.. *, .. _ Algalmudstone

~ ~ ~ ~~

Mud~tone

~M."h"ili

-.

-!..!r;_ 1 - : " " ' ; :.J: .:r ..!....,...

-

- --

water level

~ fluctuatmg

algal mats

.<~."x=~

Fluvial deposItIOn

Oohtes &

.

..

-~~-.-.-

--- ..--~- "'1'~-J;

-;

npples Hummock, ~

stranfacanon AnOXIC, cold & und turbId zone TUlbJ(iIte ~w~ ---0.. ~ and ~.~ \ane~

THERMOCLINF

Dcslccanon cracks

GeneralIzed stratIgraphIc column

Fig. 7.19: SchematiC representation of lacustrine sedimentatIOn: (A) With high sediment Input during low water level, (6) dUring high water level and (G) dUring low sediment Input (after Galloway and Hobday 1983, Fouch and Dean 1982).

@

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176

area. The Mediterranean and the Red Sea are not euxinic because they are located in arid regions where evaporation exceeds inflow of water and, consequently, the bottom waters are well oxygenated (Demaison and Moore 1980).

1<

®

250Km

>1

Surface reflux

Subsurface reflux

Fig. 7.20: (X) A-8 section across the Black Sea (Y) sketch map of the Black Sea showing a modern euxinic environment (after Demaison and Moore 1980); (Z) Model cross-section showing locales of deposition of carbonates, sulphates and chlorides in a typical euxinic basin (after Krumbein and Sloss 1963).

The conditions prevailing below an active wave base in restricted basins are acidic as well as reducing. Temperature increases towards the lake-bottom. Organic matters, settling down to anoxic bottom conditions produce a black ooze (sapropel) which is rich in ferrous sulphide. The anaerobic organisms thriving in lake-bottoms absorb trace elements such as molybdenum, copper, vanadium and uranium. The colour of the shale deposited at the lake-bottom is black due to the presence of dispersed manganese granules (Goldhaber 1978). These black shales contain much larger proportions of trace elements than any other shale (see Table 7.2). Black shales are known from a wide range of geological horizons but they are perhaps most common in the Palaeozoic and the older rocks. The world's well-known euxinic deposits include the Precambrian pyritiferous and carbonaceous shales of the Iron

177


Belt, Michigan, the Kupferschiefer of the West European Permian, the Permian Black Cherts and the Phosphoria Formation of the Rocky Mountains. In the eastern part of the Precambrian Vindhyan Basin in India, an isopach map shows a stratigraphic basining caused by a local sagging of the basement. Pyritiferous black shales (the Bijaigarh Shales) found within this depression are interpreted to have been deposited in the 'lagoonal pond' which enjoyed euxinic conditions during the Precambrian. The sandy facies overlying and underlying the shale were deposited at the lagoon fringe during transgressive phases of the Vindhyan sea (Banerjee and Sengupta 1963; Singh 1980). The trace element proportion of the Bijaigarh shale is less than that of an average black shale but the concentrations of Ni, Co, eu and Mo in these shales compare favourably with those of oxygen-free sediments (see Table 7.2). Table 7.2: Trace elements in shales and black shales (average estimates in ppm)

Blj'aigarh*** Shale*

Ba Sr Sc

Black ShaJe**

800 299 10

300 200 10

W

Li Cr V Ni Co Cu Y

Mo

160 130 21 12 38 33 0.74

100 150 50 10

70 30 10

Tn

Ga Pb Sn Zr

40 20

20 20

200

70

Shale

Sandstone

85 26 4 6 3

79 39

12 7 12 62 31 16 34

77 I

17

7 17 47

IS

2

9 29 9

22 24 17

* Analysis of average shale (Green 1959, cited by Vine and Tourtelot 1970). ** Analysis of average black shale (Vine and Tourtelot 1970): *** Analysis of average black shale and sandstone, 'Bijaigarh Sub-Stage', Vindhyan 'System'. (Precambrian) (Sinha and Singh 1964).

Recognition oj Lacustrine Facies

Morphological and sedimentological characters of large lakes and shallow ocean basins overlap in many respects. Rivers debouching into large lakes produce fan-shaped deltas similar to those bordering seas. In cross~section these show simple, steep forests overlain by flat topsets (typical Gilbert-type delta). The· finer sediments deposited at the lake centre below an active wave base are generally thin bedded and parallel laminated. Varves may develop as seasonal deposits within lakes located in a temperate climate. Storm surges produce hummocky cross-stratifications that are smaller in dimensions than those formed under marine conditions. There is no single criterion which is distinctive of a lacustrine deposit. Identification of an ancient lake is possible only by taking several of the following criteria together

178


(Pickard and High 1972). Lakes have overall circular or elliptical shapes and are surrounded by rims or barriers ('positive structural elements '). They are of limited size compared to shallow seas. In plan lake deposits show concentric patterns with gradual increase of sediment thickness towards the centre. Narrow shore and nearshore deposits surround these sediments. Mean grain-size of the sediments decreases while the proportion of organic content increases towards the lake centre. Non-marine invertebrates (bivalves and gastropods), plants, algae and stromatolites are common. Other significant characters of lake deposits are the presence of chemical and biochemical precipitates in a terrestrial setting such as evaporites, chert-banded sedimentary iron ores, freshwater limestones and marls. Finely laminated, graded turbidites at the centre; large-scale delta forests, flaser beddings in the fringe areas and occasional hurrimocky cross-stratification within the deposits may also indicate a lacustrine environment. As the coarse, gravelly and sandy sediments of the lake fringe approach the finer sediments at the lake-bottom during progradatbn, a coarseningupward sequence is produced. Such a sequence in a non-marine environment, together with some of the other features described above, point to a lacustrine environment.

Fluvial Environment Rivers originate in elevated areas and flow through flat plains before debauching into the sea or large lakes. The pattern of a typical drainage network is schematically represented in Fig. 7.21. The bigger loop in this diagram is called the contributing net. Many tributaries, big and small, contribute to the water discharge of the principal river within this net. The smaller loop is the distributing net. Distributaries flowing out of the principal stream within this loop carry water into the ocean or large lake. While doing so, they often cut through the silt laid down by the stream leaving triangular or delta shaped bodies. On leaving the mountain gorge a river deposits most of its course sediment load near the mountain front which is often faulted. Sparse vegetation and copious rainfall cause quick denudation and redeposition of the sediments in the form of alluvial fans. Humid, tropical climate is not congenial to the development of alluvial fans. Such a climate causes chemical rather than mechanical weathering and also produces dense / vegetation, which protects the slopes. In glaciated valleys paraglacial fans develop where tributaries join the main glacier. Two types of alluvial fans are recognized: dry or mudflow fan and wet fan. The former results from ephemeral streams and the latter are produced by perennial flows (Schumm 1977). On entering the plains a river can take four different patterns. In plan view these are: straight, braided, anastomosing and meandering. Of these, the braided and the meandering types are more common. Braiding is facilitated by rapid fluctuation in water discharge, high gradient, a large proportion of coarse sediment load and readily erodable banks (Rust 1978). In a braided stream mid channel bars develop due to deposition of coarsegrained sediments which the stream is unable to transport. These deposits further decrease the competence of the stream by increasing roughness of the channel. The enhanced rate of sediment accumulation, which follows, leads to the development of alluvial islands. The channel, constantly subdividing and rejoining, in an attempt to bypass these islands,

179


produce an inter-twinning pattern resembling a braid (Fig. 7.21 B). Attempts have been made from time to time to defme braiding patterns quantitatively. In one of the more recent studies a braid chatme1 is defmed as the sum of mid-channel lengths of all channels in a reach divided by the mid-line length of the widest channel (Friend and Sinha 1993). The transverse cross-section of a stream channel is dependent on its sediment load and water discharge. The cross-section, which is parabolic near the head of a stream, changes to a rectangular shape downstream (Fig. 7.21 C). This change is effected by an increase in shear stress downstream due to an increase in water discharge. The banks, unable to adjust themselves to the increasing shear stress, cave in, thereby modifying the channel cross-section (Leopold, Wolman, and Miller 1964).

@ x

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......

.....

-21 1\

( U

. . . . . /lV,

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a'

, b

b'

b~

y

Fig. 7.21: Schematic representation of fluvial patterns and cross-sections of fluvial channels (based on Allen 1965a; Leopold, Wolman and Miller 1964).

As a stream adjusts its course with respect to its water discharge and sediment load, its longitudinal profile changes gradually until it assumes a smooth (graded), slightly concave-up pattern (Fig. 7.22 A). During the process of adjustment an overloaded stream deposits a large part of its load and also straightens its course by downcutting. The limit of downcutting, called the base level oj erosion, is the sea level. When a stream reaches this base level and cannot cut further, it meanders in an attempt to decrease its gradient by increasing its effective length.


180

LongJtud1l1al Profile

©

o

,

0

000 _!'1M

~·~;

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:.;::

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. ...

; Levee

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'0

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()

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Fig. 7.22: TYPical longitudinal profile of a meandering stream showing the verucal sequence of sediments at different locations (based on Galloway and Hobday 1983) with permission of SpnngerVerlag, Heidelberg, X-Yof (A) refers to Fig. 7.21. Note 'red pattern' in dipmeter log In C.

Several other theories for the origin of meandering streams exist. Leopold and Langbein (1966) demonstrated that the sinusoidal pattern, which a meandering stream describes, is the form in which a river is required to perform the least work in turning. It is the most probable result of the processes that reduce the total energy loss to a minimum. Non-erodable clay banks and bed loads of fme-grained material facilitate meandering. Hence meandering rivers have gentler gradients than braided streams. They are also characterized by fmer sediment loads and more steady water discharge, but exceptions are not unknown. Within a meandering channel the upper layer of water moves faster than the lower layer, which is in constant friction with the bed. The fast moving upper layer of water, impinging on the concave bank due to centrifugal action, starts eroding it. As the surface water plunges downward on the concave bank, some water from the lower level emerges at the surface on the opposite bank to maintain continuity. This circulatory motion,

181


supenmposed on the downstream motlOn of water m the stream channel, generates a heltcal flow Sedtments caught up m thts flow are transported downstream to be eventually deposited by lateral accretIOn at the mner convex bends of the stream These locations, called pomt bars, are the mam areas of deposltlOn withm a meandenng stream channel Smce the pomt bars are depostted on alternate banks of the stream they help mamtam the smusotda1 pattern charactensttc of a meandenng channel (Bndge and Dtemer 1983) The deepest channel (called thalweg) also almost touches the mner concave banks on alternate stdes (FIg 723). Thalweg

avulston

- accretlOn

-

-

-

Acttve channel

Overbank deposIt Fig. 7.23: Three dimensional view of a meandering river channel snowing the morphological units.

Large transverse ndges (curvilinear ndge-and-swa1e), perpendIcular to the stream flow drrectton, were mapped by Sundborg (1956, p. 207,271) 1ll the Klara1ven River of Sweden These bars, whtch resemble the geometry of transverse dunes, and have smaller npples supenmposed on them, mtght be constdered as mCtplent pomt bars DetaIled studies on the Usn Rtver pomt bars m IndIa showed aggregates of strIpS of sedtments, whtch have been added laterally In transverse profiles these show up as senes of steps. At the same time the 10ngttudmal proftIes of these pOint bars resemble large, asymmetncal dunes WIth avalanche faces pOllltmg downstream (Purkatt, 2000). As m the dune hke bedforms generated III laboratory channels, these show accumulation of coarser grams at the toes suggestmg that the sortmg processes wtthm large pomt bars are slmtlar to those of large dunes (Sengupta et al 1999)

182


The sediments at the bed of most streams progressively fine downstream and also develop,lognormality. This means that the size distributions depict normality when plotted on logarithmic (phi) scale (Sengupta etal. 1991). Grain-size distributions within the point bars of the Usri River were found to conform to the log-normal law. After a certain distance of transportation the distribution becomes log-hyperbolic. Within each point bar there is a general tendency for the grain-size to decrease downstream. However, this pattern is not consistent along the entire course of the river (Purkait 2002). Controlled experiments in laboratory channels show that the phenomenon of grain sorting is related to the process of bedform migration. The coarser grains, entrapped at the toes of bedforms, are gradually removed from the flow, while the relatively finer ones are carried forward. Concentration of grains belonging to narrow size range in the stream leads to lognormal grain-size distribution (Sengupta et al. 1999). Fining-upward sequences develop within the point bars because the coarse-grained materials, which are the first to be deposited, are followed upward by the finer sediments (Fig. 7.22C). Lateral accretion may produce the 'ridge-and-swale' topography at the surface (Fig. 7.23). Wi~hin the channel the depth varies along its length producing a series of alternating shallows (,riffles') and deeps ('pools'). The pools are located nearer the concave erosional banks. The riffles are situated in between the consecutive pools. These features, generated by autogenic erosional-depositional processes are sometimes correlated with the mechanism of evolution of meanders (Clifford 1993). A channel is said to be meandering when its sinuosity exceeds the arbitrarily set critical value of 1.5. Sinuosity in this case is defined as curvilinear channel length divided by linear vaHey length of the stream channel. Many natural streams describe very irregular sinuous patterns. Simulation studies indicate that sinuosity of more regular patterns should be around 3.14 (Stolum 1996). LeRoux (1992), and Ghosh (2000) estimated sinuosity to range between 1.36 and 5.24 for ideally disposed symmetrical meander loops. They have also demonstrated that consistency of current (or palaeocurrent) directions in a stream is inversely proportional to the sinuosity value. From time to time a meandering channel may abandon a portion of its loop by locally increasing its slope. Chute cut-offis produced when a new meandering channel is excavated on the bar enclosed by the loop. When the abandoned loop is left as an oxbow lake, and the steam continues to flow through the shortest possible channel, it is called a neck cut-off(Fig 7.24). The deposits formed within the chute and the neck cut-off are important features of an alluvial flood basin. These prismatjc, U-shaped or triangular (in cross-section) deposits are essentially coarse-grained except in the uppermost parts. They reveal a discordant relationship with the older alluvium on the concave side. On the inner, convex margin they grade laterally to the point bar sediments. N aturallevees and crevasse splays are the other two important features associated with a meandering channel. Levees are natural embankments bordering the concave banks with slopes towards the flood basins. They are constituted of alternating coarse and fine sediments deposited by sediment-laden water overflowing the channel. During high floods the levees are breached at places by crevasse. Coarse sediments, which find easy routes from the main stream channel to the flood basin through the crevasse,


183

constItute the crevasse-splay deposIts These deposIts arc lobate or tongue-shaped In plan. Fmer gramed sedIments accumulated by the process of aggradatIon occupy the flood basIns (FIg 723) When a stream looses ItS capacIty to carry Its sedIment load, It may shIft Its course to a newer channel, WhICh IS more sUItable for sedIment transportatIOn The phenomenon, called avulsIOn, IS not to be confused wIth channel shIfts assoctated wIth chute cu.t-off, whtch tS on a much smaller scale The loss of sedIment carrymg capactty of a stream, WhICh mduces It to shtft to a newer channel, may be due to one or more of the followmg reasons (1) Decrease of the gradtent of the stream (2) Sudden lllcrease m gradIent elsewhere Wlthlll the same alluVIal plalll (3) Sudden Increases m the sedIment load and! or decrease m water dIscharge In some cases channel avulslOn has been demonstrated to ongmate as a crevasse Contmued avulsIOn develops an anastomoslllg system of channels withm an expanded crevasse splay Out of these a smgle channel Inctses the older splay deposIts and tmtiates the growth of a new alluvIal ndge (FIg 7 23) --

A

Neck cut-off

Chute cut-off

B

LEGEND

--

0- - - VertIcal accretton deposits D Rtppled, cross-lammated fine sand Q

Trough cross-bedded deposIts of the actIve rIver

D

Sand plugs

I'a: ~I

Lag gravels New channel Old channel

Fig. 7.24: Neck and chute cut-offs In meandenng channels Vertical sections of the depOSits produced In neck and chute cut-ofts are shown In A and B respectively In chute cut-off the channel IS abandoned gradually, but In neck cut-off the meander loop IS suddenly abandoned and sealed off by depOSItion of (stippled) sand plugs leaVing the main channel as an ox-bow lake Hence In the latter case the cross-laminated sandy depOSits are thin but the verllcal accretion depOSits are thick (based on Walker and Cant 1984) With permisSion of Geologica! ASSOCiation of Canada

Anastomoszng Ylver systems conStst of an Interconnected network of low gradIent channels of SImIlar order The mdlVldual channels, WhICh may be eIther straIght or smuous, are deep and narrow wIth stable banks composed mostly of Slit and clay Large, stable vegeta ted Islands wIth natural levees and wetlands separate these channels UnlIke the bratded system, where the channel IS confined to a smgle paIr of floodplain on eIther SIde, each Island of the anastomosmg system may act as a floodplaIn Noncohesive fine Slit-clay constItutIng these Islands accounts for their stablhty


184

Anastomosing channels are charactenzed by relatIvely low discharge and the islands separatmg them remain exposed subaenally m all seasons (Smith and Smith 1980) The present day braided, meandenng and anastomosmg rIver channels can be dlscnmmated in slope vs discharge diagrams (Leopold and Wolman 1957). For obvious reasons such techmques are not apphcable to ancient streams

Fluvial Facies Models Two distinct facies models are possible wlthm fluvial systems a gravelly facies associated with alluvIal fans and braided flvers and an essentially sandy facies associated with meandering systems. The characters of these two facies assemblages are summarized below. Gravelly Alluvial FacIes The facies charactenzing fans and braid-plain deposits are presented m Table 7.3. The sediments contammg at least 50% gravel have been termed coarse gramed (prefixed '0') m this scheme. Only small quantlties of sand are present in this facies Table 7.3: Facies typical of fans and braid-plain deposits (Miall 1977; as modified by Miall 1978a and Rust 1978a).

Major Facies

Om Oms

Mmor FaCies

Clast-supported, commonly Imbncate gravel With poorly defined subhonzontal beddmg Muddy matrix-supported gravel wIthout imbncatlOn or lllternal stratlfica tlOn

Ot Op

Trough cross·bedded clast-supported gravel Planar cross-bedded gravel, transitIOnal from clast-supported gravel through sand matrIx-supported gravel to sand (Sp)

Sh St Sp Fm Fl P

Honzontally stratIfied sand Trough cross-stratIfied sand Planar cross-stratIfied sand Massive fine sandy mud or mud Lammated or cross-Iammated very fine sand, silt or mud Pedogenic concretIOnary carbonate

Alluvial Fan Facies: Debns flows are characteristic of 'dry' alluvial fans Downstream decrease in grain-size and radiating sediment transportation patterns are common in these flows. An accumulatlOn of a threshold amount of loose muddy detritus is essentlal for such flows. The flows are Imtiated when an abundant water supply IS available In areas of steady ramfall the threshold amount of debns cannot accumulate because of the contmuous rather than abrupt process of erosion prevailing there (Schumm 1973) ExceptlOns to thiS rule, however, are not unknown Debris flow deposits are common m both semi-and and paraglacial environments (Rust and Koster 1984) Downstream decrease m gram-size within a fan is mainly due to a decrease m the competency of the stream The deposits change gradually from coarse gravel m the proximal part of the fan to sand m the distal part, through clast-supported, fine-grained gravel, and sand-supported gravel in the llltermediate areas (Gm -+ GP ~ Sp) A


185

'sieve texture' may be produced when the gravel lobes have a scanty supply of sand. The deposits are horizontally stratified or imbricate in the proximal parts and horizontally laminated in the distal sandy or muddy areas. When adequate proportions of mud are available in the distal areas, the clasts, being supported by a muddy matrix, may be oriented subhorizontally or even vertically (Rust and Koster 1984). Braided River Facies: Two different facies, proximal and distal, are recognized within braided rivers and braid-plains. In the proximal areas gravels (facies Gm) accumulate by vertical accretion on longitudinal bars. These are either horizontally stratified or imbricate. Next in abundance are the cross-stratified gravel deposits (Gp) with minor amounts of sand (facies Sp and Sh). The distal braid-plains have auto cyclic, finingupward sequences. These sequences have cross-stratified gravels at the base, sandstone in the middle and mudstones at the top. Many such widespread braid-plains formed in the geological past in response to tectonic uplifts. The deposits of the ephemeral Markanda River, India provide a well-studied example of a terminal fan. A network of radiating, abandoned distributaries mark the surface of this fan (Fig 7.25). The surfacial sediments of the fan consist of finjng-upward channel and overbank deposits. The erosional surfaces of the channels are alternately overlain by coarser and finer sand facies. The former is horizontally bedded or crossbedded. The latter is cross-laminated and flaser bedded. The channel sand occasionally passes onto horizontally laminated silt and massive mud facies upwards. From the proximal to the distal part of the fan there is a distinct decrease in the thickness of the channel sand. The overbank sediments consist of thin, climbing ripple-laminated sand, trough cross-laminated sand, interbedded mud, silt and sand facies passing into thick, massive mud facies with rootlets and desiccation cracks (Parkash, Awasthi and Gohain 1983). Gravelly fan deposits, sandy, and muddy-interfluve deposits have been reported from the Ganga River of India. These depict a variety of geomorphic units, wide range of grain sizes, and channel patterns along 2000 km long course of the Ganga. (Singh and Bhardwaj 1991; Singh, 1996).

Sandy Alluvial Facies Sandy Braided Stream: These sequences start with channel lag deposits at the bottom. The overlying sandy facies is cross-bedded in trough-lenticular and planar-tabular patterns. This in turn is overlain by finer grained cross-bedded and cross-laminated bar-top deposits, mostly consisting of siltstones and mudstones. Orientations of the successive sets of cross-beddings may differ widely, suggesting quick change in sediment trasportion direction. This facies model for sandy braided streams is based on observations on the Devonian Battery Point Sandstone of Quebec (Walker and Cant 1984). Coalescing bars and sand flats of braided streams often result in extensive sand sheets. These, unlike the sand deposits of meandering streams discussed below, are not confined by clays or shales. Facies of Meandering Streams: Point bars are the main depositional areas within meandering stream channels. Fining-upward sequences are preserved in point bars by the process of lateral accretion. A typical sequence, exemplified by the Devonian Old


186

Channel trench sample. Over bank trench ,amp\es

.to. •

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Fig. 7.25: Vertical sections of sedimentary structures at various locations of the Markanda terminal fan, I ndia (after Parkash et al. 1983; with permission of the International Association of Sedimentologists).


187

Red Sandstone, starts with channel lag deposits at the base. This includes gravels too coarse to be moved by the flow as bed load except during high flood, water-logged plant material and chunks of mud eroded from the river bank. Overlying the lag deposits are cross-bedded sands constituting the bulk of the lateral accretion deposits. The crossbeds are typically of the trough-lenticular type:These are overlain by horizontal laminations (plane beds) which are the products of high-flow regime conditions. There is a systematic decrease in the scale of cross-bedding towards the top but occasionally plane beds may occur interbedded with the cross-bedded units due to fluctuations in flow conditions (Fig. 7.22 C). Sigmoid surfaces (large-scale epsilon cross-beds), characterized by a steep face, flattened towards the top and bottom, may be preserved when the process of lateral accretion is episodic instead of continuous (Allen 1970a). The deposits within chute cut-offs start with a cross-bedded channel facies at the bottom. The latter are overlain by thick sequences of cross-laminated fine sand deposited during the phase of abandonment. The whole sequence is overlain by finer grained, vertical accretion deposits. In neck cut-offs thin sections of ripple-laminated fine sand are deposited during short periods of abandonment. These are overlain by thick sequences of vertical accretion deposits laid down during flood time (Fig. 7.24). Anastomosing River Facies: Six different facies assemblages have been recognized in the gravelly, anastomosing rivers in western Canada (Smith and Smith 1980). These are: 1) peat bog facies, 2) backswamp facies (silt and clay with organic debris), 3) flood-pond facies (laminated silt and clay with sparse vegetable matter), 4) levee facies (silty sand and sandy silt with rootlets), 5) crevasse splay facies (thin layers of sand with gravels) and 6) channel facies (gravel and coarse sand). Thick deposits of vertically accreted sand bounded by backswamp facies are the essential characters of anastomosing river facies_ Erosion-resistant fine sediments deposited within backswamps help in stabilizing the channel sands.

Recognition of Ancient Fluvial Deposits Ancient alluvial fans may be identified in geological record by their geometry, which is radial in plan and wedge-shaped in cross-section. The deposits are characteristically coarse and unsorted and show a progressive decrease in grain-size downstream. Organic mater is absent due to the prevailing oxidizing conditions. Disarticulated and scattered organic remains may be found. A radial sediment transportation (palaeocurrent) pattern is another characteristic of alluvial fan deposits. In the absence of suitable crossstratification, channel bar orientation, clast-size variation and pebble imbrication may serve as palaeocurrent indicators in braided stream deposits. The deposits of meandering channels are recognized by typically fining upward sequence, starting from gravelly deposits at the bottom and ending in silty or muddy sediments on top. Coarse to medium grained sediments occupy the middle portion (Fig. 7.22). The meandering channel deposits, on the whole, form shoestring sand bodies in plan. The chute-and crevasse-splay deposits are lobate, while levees occur as disconnected lenses tapering towards the flood basin. Large-scale cross-beds of trough, lenticular type are common near the bottom of the sequence. These are overlain by smaller scale trough lentic1es. Current lineation and ripple cross-bedding occur at top of the sequence. How closely the channel sands will occur within the envelop of


188

overbank clay-silt depends on the rate of subsidence of the basin and the proportion of channel-fill and overbank deposits. A slow rate of subsidence together with a large proportion 'of channel-fill sand ensures close stacking of the sand lenses in a vertical section. When the situation is reversed, widely separated sand lenses will occur in an essentially clay-silt sequence (Fig. 7.26).

Case I

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.

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1.::- : :. :1 Overbank deposit Fig. 7.26: Nature of stacking of channel sands as a function of subsidence and sediment supply. Case I: Stacking is close when the rate of subsidence is slow and the sediment supply is large (channel sand exceeds overbank deposits). Case II: Widely separated sand lenses Occur when the rate of subsidence is fast and overbank deposits exceed the supply of channel sand. (Based on Nichols

1999).

Floodplain deposits are recognized by the presence of alternating layers of thin sandstone and mudstone together with palaeosols. The latter represent 'fossilised' soils. Like present day soils palaeosols display roots, burrows and mudcracks. In the Coal Measures of Europe and North America palaeosols contain layers of peat and other organic matt~rs underlain by light coloured sandstone layers from which iron has been leached out. These palaeosols are known as 'seatearths'. Of all the ancient sedimentary facies the fluvial ones are possibly the most widely studied. These include the classical studies on the Moenkopi Formation (McKee 1954), Old Red Sandstone (Allen 1964, 1965B), Buntsandstein deposits (Mader 1985). Facies associations and palaeocurrent systems of the coal-bearing Indian Gondwana deposits have been the object of study of many Indian sedimentologists (Banerjee 1960, Casshyap, 1970, 1973; Sengupta 1966, 1970, to name only a few). In each of these studies two major facies associations were recognized: a coarse-grained sandy facies, and a finer grained facies. In the Permian Kamthi Formation of the Godavari Valley, India, the


189

former consists of closely interwoven bands of quartz wackes of both high and low matrix variety. Grain-size frequency distribution of these rocks is comparable to the typical sediments of the fluvial bed loads. These occur as thin, elongate bodies, sinuous in plan and prismatic or lenticular in cross-section. The arenaceous facies represents the fluvial point bars produced by lateral accretion in meandering channels. The majority of cross-stratifications within the sandstones belonging to this facies are of trough, lenticular type. Consistency of palaeocurrent directions within these channel sands is often fairly large (66-75%, Sengupta and Rao 1966; Sengupta 1970). The sands have gradational contacts with thin lenticles of argillaceous bands, which occur within essentially arenaceous units. The deposits constituting the finer facies of the Kamthi Formation are structureless, thinly laminated bands of clay, silt and silty sandstone having very large length/width ratio. The geometry and grain-size distribution of these sun-cracked, graded-bedded, fine-grained rocks are comparable to the suspension sediments of flood basins. These run parallel to the coarse-grained channel sandstone. The predominantly arenaceous units have intertonguing relationship with the argillaceous ones. The sequence of alternating arenaceous and argillaceous facies is repeated several times in time and space due to wandering of the river channel within its own flood plain. The pattern of sedimentation, which has evolved from this process of sedimentation, is schematically represented in Fig. 7.27 (Sengupta 2003). This diagram is designed to combine two diverging stratigraphic principles namely, the law of superposition depicting a change in the vertical direction, and the concept of lithofacies variation implying a lateral change. Such a situation is often encountered in ancient fluvial record. A rhythmic pattern of sedimentation consisting of alternately dominating channel (lateral accretion deposits) and interchannel facies (vertical aggradation) has been also reported from the Barakar Formation of the South Karanpura Coalfield by Banerjee (1960). An idealized, standard sedimentary cycle within the Barakar (Lower Permian) Formation consists of the following in a vertically upward sequence -pebbly sandstone, coarse to medium sandstone, fine sandstone, interbeded assemblage of fine to medium sandstone, siltstone and shale, carbonaceous siltstone and shale, coal and shaly coal. Several such cycles are observed. Most of these cycles display a fining-upward (sometimes partially truncated) character. The lower units are characterized by largescale cross-bedding, while the upper units contain only smaller scale cross-bedding and wavy or parallel lamination. Apparently the flow intensity progressively decreased with time and was considerably reduced during deposition of the carbonaceous unit. The presence of coal seams suggests stagnation of the depositional area during the late phase of sedimentation, allowing vegetal accumulation. This pattern of cyclic sedimentation has been interpreted as the result of channel wandering (apparently avulsion) and intermittent tectonism (Casshyap 1970). Where only immature, finegrained sediments are available for transportation, as in the Gomti River of Uttar Pradesh, India, the sediments are transported and deposited mainly as suspension load. Under this situation the sedimentary successions in point bar and natural levee deposits exhibit similar bedding structures. Small ripple cross-bedding, climbing ripple lamination, horizontal bedding are the more common sedimentary

190


structures. Scour-and-fill, large·scale cross-bedding, antIdune cross-beddmg occur only rarely under such a condition ThIck sedImentary sequences develop only where several cycles of point bar and natural levee deposits follow one another (Kumar and SIngh, 1978)

-.

Predormnantly arenaceous facIes Predominantly argIllaceous faces Fig. 7.27: Schematic representation of facies association In space and time

In a tYPically fluviatile sequence (from Sengupta 2003) With permiSSion of ElseVier

MIXED ENVIRONMENTS Barrier-Island and Beach Facies Bordering many a coastlme of the world today are rows of long narrow Islands. Between these barner-Islands and the mainland he lagoons rimmed by tidal flats. Connectmg the lagoons with the sea are tidal channels (inlets). Until about 1972 these barnerIsland complexes were beheved to have developed wholly out of progradmg barner· beach systems. Models for these systems, based mamly on studies 10 the Galveston and Padre Islands, Texas, were also formulated (Dlckmson et a/ 1972). More recent studies show that the barner-island complex IS a composite depositional system IOvolvmg several dIscrete envIronments-beach, tldallnlet, tIdal delta and lagoon (FIg 7 28) A smgle generahzed model cannot do justice to such a complex system Four separate faCIes models, applIcable to the barner-beaches, tidal mlets, tidal deltas and lagoons, are dIscussed below, followmg Remson (1984) TIdal Flat faCies IS dIscussed separately


191

Barrier-Beach Facies The barrier-islands bordering coastlines have shorefaces (beaches) on the seaward side. Four subenvironments of sedimentation are possible withm the barner-beach system. These are: shoreface, foreshore, backshore-dune and washover flats (Figs. 7.28 & 7.29). Tidal inlets, tidal delta, and lagoons connecting barrier beaches and tidal flats are discussed under separate heads.

Fig. 7.28: Block diagram showing various subenvironments in a barrier-beach complex (after Reinson 1984). With permission of Geological Association of Canada.

Shoreface

Shoreface extends from the low-tide mark to a depth of about 10-20 m. Three zones of shoreface-lower, middle and upper, are recognized. The middle shoreface, which extends up to the breaker zone, is dominated by wave energy. The typical deposits in this zone are highly bioturbated, fine to medium-grained, clean sand, with silt and shell layers. Seaward-dipping, wedge-shaped, planar laminae and landward-dipping ripple laminae are the common sedimentary structures in this zone. Trough crosslaminae may also be present. Occasional storm surges may drastically lower the wave base in the middle shoreface, putting much of the sediment in suspension and subsequently depositing it seaward. This phenomenon is believed to be responsible for generation of the hummocky cross-stratifications found in geological record. In the lower shoreface shelf and basinal processes are predominant although some wave action is also present. Plane beds of bioturbated. fine to very fine sand, with layers of silty mud are deposited in this zone. The degree of organic activity decreases towards the upper shoreface where the deposits are coarser. The complex hydraulic condition of the surf zone produces multi-directional trough cross·bedding. Ridges (bars) and runnels (troughs) develop on the upper shoreface when the sediments transported from the beach to the shoreface are returned and redeposited at the edge of the foreshore under quieter conditions.


192

Foreshore

Foreshore is the area exposed between udes It has a steeper gradIent than the surf zone. The deposIts here are well sorted by swash-backwash action Thmly lammated, wedge-shaped beach deposIts dIP gently towards the sea In some beaches berms separate fore shores from backshores (FIg 729).

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Backshore Backshore occurs on the landward sIde of the foreshore. The sediments of the backshore mc1ude lag deposIts and heavy minerals These dIP very gently towards the land. Eoban dunes wIth planar and trough cross-stratifications occur closer towards the land Rootlets and burrows are common withm these dunes. Washouer Flats

Thin lobate sheets of sand which flow over foredune barriers during high strom surges and extend Into lagoons are called washovers (FIg 7 28). These consIst of medIum to coarse sand and, rarely, gravels. Parallellammation IS the usual structure of the washover deposIts but small-scale delta'foresets may form when wash overs plunge into lagoons

Tidal Inlet Facies TIdal mlets are narrow deep channels connectmg the lagoons with the open sea (Fig 728) Water flows v1gorously through these channels dunng flood and ebb Lateral mlgratlOn of the tIdal mlets causes the tIdal deposlts to accumulate as m a rrugratmg stream channel The sand body produced out of thIS process of m1gratlOn 1S elongate parallel to the trend of the bamer 1slands Its thickness IS of the same order as the depth of the inlet The depos1ts withm the mlets show fmmg upward sequences, starting WIth lag gravels at the bottom and endmg ill finer sedIments at the top. The cross-beds WIthin these


193

deposits carry imprints of flood and ebb flows and are naturally bi-directional. Plane beds and ripple laminae occur within the upper deposits. Five major depositional units, starting from lag gravels at the bottom and ending in fmer sand - and seaward-dipping laminae at the top, were recognized within the Fire Island Inlet, New York state (Kumar and Sanders 1974). These are the channel floor, deep- and shallow-channel, spit-platform and spit deposits. Each of these deposits is associated with a particular type of sedimentary structure and texture. This model should work broadly for all tidal inlet deposits.

Tidal Delta Facies Tidal deltas generate at either end of tidal inlets by tidal flows (Fig 7.28). The ebb-tide deltas, produced at the seaward end on the tidal inlet, are modified by wave action and longshore drifts. The flood-tide deltas occur at the landward outlet of tidal inlets. The deposits of tidal deltas are essentially of sand size. The structures at the base are bidirectional megaripples. These are overlain, in the case of flood deltas, by landward dipping, large-scale ripples with planar-tabular and trough-lenticular internal structures. The cross-bedding set thickness decreases upwards. In ebb deltas, complex bedform patterns may be produced as a result of interaction with ocean waves. Lagoonal Facies

The lagoonal deposits consist of alternating sandy and silty facies. The former is found in channel fills and washover sheets. The latter is produced within the lagoon under subaqueous conditions or derived from the adjacent tidal flats. The sandy deposits are rich in organic content. These are overlain by coal and peat produced within marshes and swamps at the margins of the lagoon.

Sedimentation in Mixed ~nvironment: Indian Examples Recent continental ~p.d marginal marine facies of the Godavari delta on the eastern coast of India provide ad example of a mixed environment of sedimentation. The GautamiGodavari River channel with its secondary distributaries forms a part of the subaerial topset beds of the Godavari delta. The two major distributaries of the Godavari River are interconnected by a reticulate system of tidal creeks. These creeks pass laterally into man$l'0ve swamps. The backwater is a vast shallow lagoon, separated from the open sea on the south by a narrow barrier beach and on the east by a composite system of swamps, tidal creeks and beach. At least five barrier islands occur off the Vrudha-Gautami River confluence. These narrow, elongate islands run parallel to the mainland beach. The sands of the offshore barrier islands of the Godavari delta are very well sorted and have an almost symmetrical grain-size distribution. Those of the mainland beach are moderately sorted and negatively skewed. The dune sands are well sorted and positively skewed. The silt and clay proportions are lesser in the beach and higher in the dunes (Naidu 1966). These observations provide clues to the identification of the subenvironments in a ancient barrier-beach complex. The Proterozoic Vindhyan exposures of central India are arranged in several linear facies belts. Each of the facies in lithology, sedimentary structure and palaeocurrent pattern corresponds to a particular unit of sedimentation within a barrier-coastline model (see Banerjee 1974 for details). The Vindhyan sea was recognized as microtidal

194


(tide range < 2m), occasionally rising to mesotidalleve1 (2-4m). On the open shelf to the north-east a shore-normal, bipolar tidal pattern was identified, while within the embayment the pattern was much more complex. Strom surges, wind and wavegenerated currents added to the complications (Banerjee 1980). The parallel and cross-laminated, grey-black, pyrite-rich shales and siltstones of the Bijaigarh Formation (Kaimur Group) within the Vindhyans were interpreted as Proterozoic lagoonal deposits, while the micritic and dolomicritic lime-mud-bearing Bhander Limestone was thought to represent an ancient carbonate tidal flat (Banerjee 1974 ; Singh 1980, 1980a). Ripple-drift cross-laminations with rhythmically sizedifferentiated foreset laminae, believed to be characteristic of tidal environment, were also reported from the Bhander Formation (Chanda and Bhattacharyya 1974). Although these observation.s point to a barrier-coastline, tidal or lagoonal condition of deposition, alternative interpretations, recognizing at least a part of the Kaimur Group of rocks within the Vindhyan Supergroup as fluviatile deposits, are also known CAuden 1933 ; Bhattacharyya and Morad 1993). Stratigraphic Sequences in Mixed Environments

Stratigraphic sequences developing out of regression (progradation), transgression and migration of barrier-island systems are broadly as follows (Fig. 7.30 A-C after Reinson 1984).

Regressive Model A typical sequence developing out of regression of a barrier-island system has finer shoreface deposits at the bottom, grading gradually to coarser dune sands at the top through foreshore and backshore sediments (Fig 7.30 B). Abundant sediment supply is an essential condition for preservation of these deposits. This condition is satisfied in an inter-deltaic coastal setting. In fact, many a barrier-island sequence in stratigraphic record are associated with deltas. Barrier Inlet Model This is a fining-upward sequence with a thinning-upward trend in cross-bedding set thickness. It has an erosional base and is dominated by sand facies of tidal-channel and marginal spit-beach environments (Fig. 7.30C)_ Transgressive Model The facies sequence is variable, depending on the local condition. It is generally characterized by subtidal and inter-tidal back-barrier facies and does not show a defmite fming or coarsening-upward facies. In the regressive model the lagoonal facies overlie the sandy facies but in the transgressive model they underlie or are incorporated into the lower part of the sequence (Fig. 7.30A).

Tidal Flat Facies Tidal flats develop in coastal areas where~wave action is not strong. Within the barrierisland system such an environment prevails near the lagoons (Fig. 7.28). The tidal flats


195

of northern Europe, which are sheltered from the wave action of the North Sea by a row of barrier islands, may be cited as examples. Tidal flats may also occur on open sea coasts where large width and low gradient of the beach dampen wave action. The Mailiao Flat of Taiwan, the Inchon Bay of Korea and the Chandipur Flat on the eastern coast of India are examples of open-to-the sea tidal flats. Tidal flats can be divided into two groups, depending on whether the constituting materials are essentially siliciclastic or carbonates.

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Fig. 7.30: Facies models of barrier island stratigraphic sequences (based on Reinson 1984). See text for details. With permission of Geological Association of Canada_

Siliciclastic Tidal Flats Fine sand, silt and clay constitute the bulk of the siliciclastic tidal flats. The nature of sediment distribution in these flats largely depends on local geographic and climatic conditions. In the sheltered intertidal areas bordering the North Sea a mud flat composed of mud, fecal pellets and thin bands of sand occurs on the landward side of the flat. The proportion of sand increases towards the sea until the flat becomes wholly sandy near the sea coast. In-between the mud and the sand flats occur mixed flats composed of muddy sand. The biogenic content of the flat increases as one moves from land to the sea. A generalized cross-section across an open-to-the-sea tidal flat based on observations at St. Michel, France is shown in Fig. 7.31. The surface structures in sheltered intertidal zones, like rain and hail imprints, desiccation cracks and bird tracks, indicate periodic subaerial exposure. Horizontal lamination in sandy flats and parallel lamination in silty flats are common. Combinations of straight-crested, bifurcating (in plan), tuning fork-shaped ripples,

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197

interfering ripple systems produced by current and wave movements, and lenticular flaser beddings are some of the important characters. The subtidal zones, which remain submerged during low-water, springtide level, are the sites of deposition of coarse sediments. The gullies traversing these flats contain coarse, channel-lag deposits. Megaripples and flaser beddings are found here. Escape traces of organisms are common but shells, in orientations which they occupied, when alive, occur rarely. The supratidal areas located on the elevated landward side of the tidal flats normally remain exposed to the atmosphere. They are inundated only during exceptionally high spring tides. Salt marshes consisting of clayey sediments and occasionally peat are common in the supratidal areas. Coarser sediments are restricted to the channels traversing the marshes. The clayey sediments are often stabilized by growth of saltresistant plants within the marshes.

Carbonate Tidal Flat The carbonate tidal flats do not differ much from the siliciclastic flats except in composition. Based on studies in such classical areas as the Bahama Banks of the USA, Shark Bay of Western Australia and the Persian Gulf, James (1984) recognized two major carbonate flat facies: those associated with high-energy conditions and those related to low-energy environment. A summary of the characters of tidal flat carbonate facies based on James (1984) follows. The intertidal carbonate deposits are characterized by the presence of algal mats, irregular or even laminations (cryptalgal laminations) with fenestral porosity and desiccation cracks. The algal mats are leathery sheets of blue-green algae growing on top of sediment surface. The growth of these mats is controlled mainly by rainfall, moisture, salinity and the presence of other organisms. Algal growth is luxuriant in areas of high rainfall and hypersalinity. In areas of normal salinity the algal growth is deterred by gastropods which feed on them. In areas of high rainfall the algal growth may extend even up to the supratidal areas. Elongate, sheet-like cavities (designated as 'laminoidfenestrae', 'loferites' or' bird's ryes') left by growing algal colonies are preserved in the geologic record even after the algae have decayed. These allow identification of ancient algal colonies. The presence of algae can also be inferred from the occurrence of stromatolites-stratiform, crenulated, and arched organo-sedimentary structures produced by sediment-binding algae (see chapter 6). Desiccation cracks are created by subaerial exposure of the algal mats. The cracks between the desiccation polygons are filled by lime mud. Sediment characters of the different parts of an ideal carbonate tidal flat are as follows.

Lower Intertidal Zone Burrowed, bioturbated sediments, desiccation polygons and luxuriant growth of algal mat characterize this zone, particularly when the condition is hypersaline. Oolites formed from aragonite needles and organic matter in the shallowest sea water, may be moved out of place by strong tidal currents and deposited in the lowest intertidal zone.

198


Middle to Upper Intertidal Zone Sediments are grey-brown due to prevailing oxidizing condition. Algal mat growth is common. Fenestral pores and desiccation polygons, indicative of prolonged exposure, are present. Burrows of worms, crabs and crustaceans, and rootlets of salt-tolerant plants are common features. Supratidal Zone Surface crusts ('hard grounds') produced by lithification of sediments due to prolonged subaerial exposure are common. 'Teepees' are produced by pushing up of the broken polygons on surface crusts due to the force of crystallization or penetration of plant roots. The clasts are often cemented by carbonates. Caliche, algal mats (in areas of high rainfall) and evaporites (in arid areas) are common features in some supratidal areas. The nature of carbonate and evaporite precipitation is largely dependent on the chlorinity of the ground water and the amount 0f freshwater flushing in the supratidal zone. Stratigraphic Sequences in Carbonate Tidal Flats

High-Energy Carbonate Flats The characters of high-energy carbonate flat 'deposits are essentially the same as those of the siliciclastic beaches-coarse-grained, poorly sorted carbonate sand with lime mud matrix in the lower foreshore, and well-sorted lime sands in the upper foreshore. The lower foreshore has small- to large-scale trough cross-bedding, oriented parallel to the shoreline. The upper foreshore has seaward-dipping, planar cross-beds. These sediments may also have 'keystone vugs' or 'microcaverns' (similar to fenestral structures) produced by gas escape. These are almost invariably cemented at a later stage. A typical vertical sequence formed'in a high energy intertidal situation is shown in Fig. 7.32 C. The sequence, which starts with a lime-sand conglomerate at the bottom, is overlain successively by argillaceous limestone and bioturbated mudstone, to be capped, eventually by calcrete. Low-Energy Carbonate Flats Vertical sequences are developed either by progradation of the carbonate tidal flats or by shoaling of carbonate sands of barrier islands. The basal units of such sequences are composed of coarse grained deposits of the surf-zone. These are overlain by bioturbated wacke-stones and packstones with marine fauna and stromatolites of the subtidalintertidal areas. The subtidal deposits are well sorted, oolitic, pelletoidal and skeletal lime sand occasionally with oncolites. Planar bedding and herring-bone crosslamination, with set thickness decreasing upward and ending in small-scale ripples, on top, are the major sedimentary structures. The structures are often destroyed by worm burrows. In shoaling-upward hypersaline sequences, as in the Shark Bay of Western Australia, stromatolites may abound on top of a skeletal metazoan bioherm or biostrome (Fig. 7.32 A and B). The deposits of the supratidal zone which overlie the subtidal-intertidal facies are evenly laminated. Flat-pebble breccia and desiccation polygons are common features in these deposits. Authigenic evaporates develop in arid regions flushed by hypersaline


199

ground water. This is followed by precipitation of dolomite due to a rise ofMg++ ICa H ratio of ground water and subsequent dolomitization. Collapse breccias, produced by partial solution of evaporates, may cap the sequence (James 1984). LOW ENERGY INTERTIDAL Sandy/Grainy Muddy Sequence Sequence

HIGH ENERGY INTERTIDAL

®

©

.'

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.~~J~

.

','~.:

Desiccation polygons Even laminations ~

-

Ske1etallime sand

....

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Hardground

-

==6~

. ~.'.

Argillaceous Lst. Skeletal & intraclast Lime sand/ conglomerate

~

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~ Lamination & fenestra

1~:,:-·.r.r"l Skeletal lime sand

It'. ,IJJ

H

Mudstone. fossiliferous, bioturbated

Limestone, argillaccous/sandy

~ Lime Sand/Conglomerate Fig. 7.32: Hypothetical sequences developed on (A) low-energy (muddy) subtidal unit, (8) high-energy (grainy) carbonate sand unit, and (C) high-energy intertidal carbonate unit (based on James 1984). With permission of Geological Association of Canada.

Tidal Flat Sedimentation: Indian Examples The Chandipur tidal flat situated on the eastern coast of India provides an example of a beach-related siliciclastic tidal flat bordering the open sea (Fig. 7.33). This tidal flat enjoys tropical climate. The mean tided range lies between 2 and 4 m; hence the setting is designated as macrotidal. The western part of the intertidal zone enjoys a non-barred setting, while several sand bars occur on the eastern part. Nearer the land the non-barred planar flat is composed of laminated sand frequented by burrows of ocypodid crabs. As one moves towards the sea the laminated sand grades to silty sand which is occasionally bioturbated by polychaetes. Nearer to the sea occurs a more silty facies. The silty sand has ripple-drift lamination,


200

scour and fill, flaser beddlllg and clImbmg ripple lamlllatlon. The sllty facIes has ripple-drift lamInatIOn, scour-and-fill structures and hummocky cross·stratIficatlons (HCS) The HCS develop only dunng monsoon weather frequented by cyc10mc storms In the barred flat lam mated sand, meganpple lammatlOn and flaserbeddmgs predomlllate over other structures (Mukherjee, Das and ChakrabartI 1987) The Rann of Kutch, located on the western coast of IndIa, enJoys mtertidal-supratlda1 enVlfonments The extensIve Rann (- 30,000 krn 2 In area) IS flooded for about three months every year by sea-water 2m deep as a result of monsoon storm surges Fme sut, clay, gypsum and a small amount of carbonate constItute the deposIts m this Rann (G1enme and Evans 1976). CALCUTTA.

---------1. 8 A :y

°rv , LEGEND

rn 0

500m

L--...J

~ ~

RIVer mouth shoal

IIIIIIUIII

IntertIdal sand bars

~ [L!] IT]

Neck cut off

0

Coastal dune

[:i::J

Manne terrace (upper)

Manne terrace (lower) Marsh land FluVlO manne terrace Lagoonal c1 ay

Fig. 7.33: Geomorphic units within the tidal flat around Chandlpur, India (after Mukherjee et al 1987. Chakrabartl 1991. unpublished DST report)

Deltaic Environment

A delta IS defined as 'a deposit, partly subaenal, built by a river into or agamst a permanent body of water' (Barrell 1912) The morphology of a delta depends on the ra tlO of sedIment supply and the energy of water transporting and deposltlllg it. When the sedIment supply IS large, deltas appear as protrusIons of shorelmes into the sea or large lakes Arcuate, cuspate, lobate, and blrd·foot deltas develop progressIvely as the sedIment supply IS mcreased with respect to the wave energy. When the proportIOn of sediment supply IS very low, the erosive power of wave energy predommates and Indentations are produced on the coast In the form of estuaries (Fig. 7.34)


.. .. '

,

201

"

BIRDFOOT

HIGH

LOBATE

CUSPATE

ARCUATE

..... . . . . .

.

.

.

"...

. . . .

..

..

"ZERO"

"NEGATIVE" (ESTUARY)

LOW

. .' .' :1

Fig. 7.34: Diagrams illustrating changes in shapes of subaerial deltas depending on wave energy and sediment input (after Curray 1975. In A.G. Fisher and S. Judson 1975. eds.).

As early as 1885, G. K. Gilbert recognized that topset, bottomset and foresetbeds are formed when streams debouch into still bodies of water in lakes. The slope of the foreset, under such a condition, is guided by the angle of repose (- 30°), whereas the top- and bbttomsets remain nearly horizontal. Since 1885, deltas showing well-demarcated top- , bottom- , and foreset units have come to be known as Gilbert-type deltas. In deltas of the classical Gilbert-type it is the foreset that characterizes the delta, the top- and the bottomsets being extensions of the fluvial and lake-bottom sediments respectively. The foreset in a Gilbert-type delta consists of coarse sediments deposited suddenly as the flow of the stream is deterred on reaching the quieter body of the lake. The foreset remains undisturbed only when the water level in the lake is stable for a fairly long period of time. When the water level fluctuates irregularly, complex patterns may develop due to interfingering of the top-, bottom- and foresets. When the level of water changes in a series of steps, the delta may take a step-like form. In cases of extremely trregular fluctuation of water level, the foresets, which characterize the Gilbert-type delta, may be completely obliterated. The whole delta deposit in such a

202


case is represented only by a thick wedge of sediment interfingering with the coarser fluvial sediments on one side, and the finer lake bottom deposits on the other side (Dunbar and Rodgers 1957). Deltas of the Gilbert-type are produced only when the densities of the effluent water in the stream channel and that in the sea or the lake into which this water is released are of the same order, allowing uniform mixing in three dimensions (Busch 1974). When the water in the stream is less dense than the water in the embayment, a sheet flow results because the mixing now is only 'two-dimensional'. When the magnitude of sediment discharge is small, an arcuate sand bar is deposited at the mouth of the stream. When the discharge is high, a cuspate or bird-foot type of delta may result. If the effluent water in the stream, because of its high suspended load, has a much higher density than the water in the ocean, a channel is eroded on the continental shelf in the form of a submarine canyon (Bates 1953, see also Fig. 7.34).

Classification of Deltas Deltas have been classified by Galloway (1975) on the basis of relative influence of the fluvial, oceanic and tidal processes which operate during delta building (Fig. 7.35). The facies characters of these three types of deltas are described below, following Miall (1984a). Many natural deltas, it must be realized, display characters which are intermediate between these three ideal end-members.

River-Dominated Deltas As a river debouches into a static body of water its competence decreases and the coarser sediment load is dropped at the fringe of the sea or lake in the form of spits, bars and islands. The distributary channels cutting through the alluvial plain are bordered by levees. When the proportion of suspended sediment carried by the stream is high, the river mouth is extended towards the sea by sedimentation. The deposits are oriented at a high angle to the coastline to form bar-fInger or shoestring sand bodies, as in the bird-foot delta of the Mississippi River (Fig. 7.36A). The space between the distributary channels of such a delta is filled by marshes. At a high stage of flooding the distributary channels may be breached by crevasses. Crevasse splay deposits are formed when coarse sediments, pushed out of the crevasse, splay out on the adjoining marshes. Unstable conditions may arise when the rate of sediment deposition is exceptionally high in a delta. Frequent failures on the seaward side of the delta in such a case might result in faults contemporaneous with sedimentation. Such faults are called 'growth faults'. A lobate, fan-shaped delta is produced when a heavy load of sediment is discharged by a stream on a plain of low gradient. In the Mississippi delta for example, a major delta lobe was created every time the Mississippi River discharged its sediment load towards the sea. The lobes thus produced are preserved even after the distributary channels responsible for their sedimentation abandoned their earlier courses to take a shorter one to the sea. Seven such deltaic lobes have been mapped in the Louisiana coastal area (see Fig. 7.37). While deltaic sedimentation was going on along the Louisiana coast, the sea encroached on the land on several occasions to flood the delta surface. Complex patterns of flUVIal, fluvio-marine, paludal and marine sedimentation developed within the


203

Mississippi delta out of the interaction between the delta and the sea. The fluvial sediments were deposited in stream channels, natural levees, point bars and abanuoned stream courses. Brackish fluvio-marine deposits were laid down at the mouths of distributary channels. Paludal sedimentation took place in marshes, swamps, tidal channels and lakes. The marine sediments were confmed to beach, foreshore and deeper water reefs (Kolb and van Lopik 1966). SEDIMENT INPUT

Nile WAVE Niger DOMINATED

Mekong

TIDE DOMINATED Colorado

Brazos WAVE ENERGY FLUX

Copper

Ganges-Brahmaputra TIDAL ENERGY FLUX

Fig. 7_35: Variation of shapes of deltas as a function of sediment input, wave energy flux, and tidal energy flux (after Galloway, 1975).

Wave-Dominated Deltas In areas of strong wave action the delta sediments are continuously reworked and the sand bodies are reoriented into a series of beach ridges paralleling the coastline. These are called 'strike-oriented' ridges. In areas dominated by longshore drifts the entire delta assumes an asymmetric ('skewed') shape. The delta of the Nile is a typical example of wave-dominated delta (Fig. 7.36B). The Rhone delta, perhaps the best-studied wave-dominated delta in the world, has a thick sequence of prodelta shelf mud at the bottom. This is overlain by delta front sand, often called 'coastal barrier sand' (Galloway and Hobday 1983) Stacking and coalescence of individual sand bodies here resulted to delta front sand sheets of enormous thickness. The thickness has further increased by growth faulting_ The delta front sand dips very gently towards the sea and shows a larger degree of marine influence than the sands of river-dominated deltas. The wave-dominated delta front sequence coarsens upward like the sequence produced by progradation of a marine shoreface. In areas influenced by strong winds the coarsening-upward facies is covered by finer grained, well-sorted, coastal dune sands. The uppermost part of the Rhone delta sequence is criss-crossed by fluvial distributaries.


204

A X

Marsh

Y

...

~

A 'j.'-,,(,o

Silt&l:~~~~':':'::-':"""-~<~ Clay _._.- ..... -'-'_.-.. - ._..-~

_.- '-'-'--

Miles

-'-

~·-·--,-·-·-B·-·-·-'-

~c.

\'

A Clay

~ .~ ..

:~~. '~"

.

...

'

B

y

~

~

D'

Mean Sea-Level

Distributary Channels / \

M =:--,

SO;;;;::::::;:;;'

,c==:"........ S 2 5 2

High water level

t:t.

N

Low water level

Bay

o f

Bengal 32 ,

Miles

Fig. 7.36: Deltas in plan and profile. (A) the Bird-foot delta, Mississippi, dominated by fluvial action, (8) the Nile delta, dominated by wave energy, and (C) the Bengal delta-estuary system, dominated by tidal energy. (Based partly on Smith Jr., 1966).


10

10

,

I

20

205

30 MJ!e,

Fig. 7.37: Deltaic lobes of the MISSISSIPPI River (after Kolb and van Loplk 1966) The flow lasted for 500 to 1000 years In each case ApprOXimate time of flow before 1950 (1) around 5000 years, (2) around 4000 years, (3) around 3000 years, (4) around 2000 years, (5) and (6) between 2000 and 1500 years, (7) about 500 years to present day

TIde-DomInated Deltas

TIdal currents, hke waves, rework the deltaIC sedIments and rearrange them mto a senes of elongate bars extendmg ('dIp-wIse') from the channel mouth to the subaqueous delta platform. ThIs platform IS produced by deposItIOn of mud, slit and muddy sand, wh1ch are earned downstream by suspensIOn currents form the prodelta depOSIts Estuanes develop when the dlstnbutanes find outlets mto the sea through funnel-shaped channels. On land these t1dal estuanes may take smuous forms, cuttmg across ttdal flats and supratIdal marshes (F1g. 7.36C). The vertlcal sequence m the Colorado estuanne channel, one of the well-studIed tldal deltas of the world, starts w1th a basal scour filled wlth lag depos1ts This 1S overlam by well-sorted channel-fill sand contammg mud, s11t drapes, clasts, macerated plant debns and shells The sedlmentary structures m th1s sequence mclude npple lammatlOns and cross-beddmgs wlth b1-d1rectlOnal d1ps Laterally, the coarse-gramed estuanne sed1ments grade mto finer delta plam deposlts The sedlments m the tIdal current sand ndges coarsen upward The bottom of the sequence IS occupIed by mud of prodelta shelf fac1es These are overlam by sand wIth mterbedded mud lenses R1pples, crossbeddmg and parallellammatlOns are the common sedlmentary structures. The sequence IS capped by finer gramed t1dal flat deposlts The hIstory of sedImentation m the Bengal Delta, the largest tlde-doffilllated delta m the world, wlth an area exceedmg 59,000 sq Ian IS comparable to that of the Colorado


206

delta. The Bengal delta had its origin in the Oligo-Miocene when deposition of a thick sequence of clay and silt (the Bhagirathi Group; Biswas 1963) took place on the Eocene shelf limestone of the Bengal Basin. The total thickness of these deltaic deposits in the south-eastern part of the Bengal basin exceeds 5 km (Fig, 8.21, 8.22). The thickness of the Bengal deltaic sediments decreases north-west, near the fringe areas of the basin. Marine influence decreases and the section becomes sandier as the basin fringe is approached. The present delta, covering large portions of West Bengal (India) and Bangladesh, is occupied by tidal flats and marshes which are travesed by the distributaries of the Ganga (Hugli River) and the Brahmaputra Rivers. (Fig. 7.36C).

Recognition of Ancient Deltas Being the largest repository of clastic sediments, deltas are well represented in stratigraphic record. Ancient deltas may be identified by the presence of a thick wedge or lobe of shallow-marine to lacustrine clastic sediments showing typically coarseningupward sequences. The wedge grades on one side to sediments of deeper water facies and on the other side to non-marine sediments. The character common to delta deposits is an upward-coarsening progradational sequence leading to a marine, delta margin sand unit. This sequence is overlain by deposits of an aggradational facies - distributary chall)1el sand and crevasse-splay sand bodies. The typical vertical succession in such a delta, as recorded by Galloway (1975), is: Top

Bottom

4. Open marine shelf mud or carbonates 3. Thick sand body separated from the coarsening-upward sand body by thin layers of mud. 2. Typically coarsening-upward sand body which thins and pinches out laterally 1. Prodelta mud blanket

In some cases the sandy zones (2 and 3 above) are repla€ed by a single, thick sand body which is narrow and elongate in plan and shows a fining-upward profile. In all cases the delta platform and delta plain deposits are criss-crossed by distributary channel sands. These lens-shaped sand bodies show fining-upward sequences.

8 TECTONICS AND SEDIMENTATION

SEDIMENTARY BASINS SedIments exposed to atmosphere are ltkely to be eroded soon after their depositIOn by vanous natural agencies For preservatIOn m the geological record It IS essential that the sediments be pushed below the base level of erosIOn The preservation potential of a deposit IS determmed by the rate of subsidence of the area of depOSItion Vis-a-VIS the rate of sedImentatIOn The areas of the earth's crust which smk rapidly to allow preservatIOn of a thick sediment load are called basms Basms occupy large areas measunng thousands of square ktlometres Thmnmg of sedIment layers towards the baSIn frInge IS a sure mdlcatlon of subSidence concomItant with sedImentatIOn In modern tectOnIC parlance a basm IS a 'prIsm of rock formmg a thick sedImentary succession' (DIckmson 1974) A model for baSIn evolutIOn must explam, not only the mechamsm of mitlatlon of a depreSSIOn m the earth's crust but also the process of uphftmg of the adjacent areas, m order to prOVide a source for the sediment fill BaSIn subSidence was long Imagmed to be the effect of gravltatIOnalloadmg durmg sedimentatIon PreCIse computatIons have ~hown, however, that the crust of the earth would not buckle to the extent needed for generation of large sedImentary baSInS due to the weIght of the sedIment load alone, but thIS could happen ill the case of smaller bas illS For more than a century the ongm of large sedimentary baSInS was explamed III terms of the claSSical geosynclmal model Smce the advent of the plate tectOnICS concept m the mid-SIXties of the 20th century, there has been an equally enthUSIastIc attempt to explam the ongm and development of large sedimentary basms m terms of plate tectOnICS These Ideas are dIscussed below m the chronologIcal order In which they evolved GEOSYNCLINAL CONCEPT The geosynclmal concept had ItS ongm from the dIscovery by James Hall m the mld19th century that the thickest sediment accumulatIOn m the Appalachians IS along the line of maxImum depreSSIOn (Fig 8 I A) It was recoglllzed that large-scale sedimentation, whIch eventually led to the development of mountaIn belts, occurred along lmear troughs along the contmental borders These troughs, separated from the ocean basms by elevated areas (,borderlands'), were deSIgnated 'geosynclines' by Dana m the late 19th century A geosyncltne need not always be restncted to the contmentaloceamc boundary The Alpme-Hlmalayan belt, an area of thick sedimentation and mtense orogemc act lVl ty, IS located between two contlllental blocks Hence modern 207


208

workers define a geosyncline as a basin of sedimentation located marginal to, or between, two cratons (Aubouin 1965). Cratons are the tectonically stable areas constituting the nuclei of the continental crust, while geosynclines are the locales of intense tectonic and igneous activity, both syn- and post- orogenic. The essential implication in the concept of geosyncline, therefore, is an idea of mobility. Mobility of the earth's crust in certain long, linear belts was imagined to be responsible for the accumulation of an unusually thick sequence of sediments. A typical geosyncline of the above-mentioned model consists of an amagmatic zone of relatively shallow-water sedimentation bordering the craton. This zone, which receives sediments mainly from the cratonic source, has been termed the miogeosyncline. Away from the craton lies the deeper part of the geosyncline, the eugeosyncline. Separating the miogeosyncline from the eugeosyncline is a ridge, originally termed the geanticline but renamed the miogeosyncIinal ridge by Aubouin in 1965 (Fig. 8.1 B). The eugeosyncline receives sediments from the geanticline on one side and from the tectonically active borderland located on the other side. In marked contrast to the miogeosynclinal deposits, the eugeosynclinal sediments contain abundant tuffaceous material, the products of volcanic activity of the island arcs bordering the eugeosync1ines. The eugeosync1inal sediments are also permeated by basic flows erupting on the ocean floor in the form of pillow lavas. The eugeosync1inal sediments of extraordinary thickness, traditionally related to orogenic events, are broadly classified into two groups, pre and post-diastrophic. Since the nineteenth century the former has been designated as flysch and the latter as molasse.

~ ro~~~o

~~

o

MIles

IntruSIves

50

Tectomc ndge

Kay (1951)

Craton

MlOgeosynchne

Auboum

>I< )1<

Eugeosynclme

---7\

(Borderland)

Mlogeosync1mal

(1965) Foreland

+

ndge MlOgeosyncJinal furrow

>/<

)1<

Eugeosyncline furrow

~ EugeantlclInal ndge

~+~~+~~+~~;:~----------~~~~~=-----------~1

®

Tectomc Land

Contmental Crust

f+.+ +

OphiolItes Slahc Basement

Oceamc crust

-E-- DIrectIOn of Orogemc MIgratIon and Overturmng

Fig. 8.1: Geosynclinal termin.ology (A) after Kay 1951, (8) after Aubouin 1965. (A) represents restored cross-section across the Appalachians from Eastern New York to Maine. The deposits are mostly CambrO-Ordovician in age.

Tectonics and Sedimentation

209

Flysch has traditionally been described as a thick succession of argillaceous sediments of deep-water origin interrupted by dark gritty sandstones. Pillow lavas (altered to 'greenstones') ,rre common within this sequence. Many flysch deposits start with massive graded beds at the bottom and end in pelitic layers on top through a sequence of parallel lamination and rippled beds (the Bouma sequence). The presence of such sequences in beds of deep-water origin, as evidenced by deep-water biota, led to the conclusion that the dark sandstones sandwi'ched between the argillaceous sequences were transported and deposited by turbidity current activity. The shale-siltstone-fine-grained sandstone-graywacke sequence of the Simla 'Series' (Precambrian) of the -Himachal Pradesh and Garhwal Himalayas were interpreted as flysch (Sen, Bhattacharya and Ray 1969; Valdiya 1970). The Precambrian metasediments of Ghatsila in eastern India consisting of a rhythmically bedded sequence of quartzites and mica schists, originally deposited as a thick sequence of sandstone and shale, bear all the imprints of a typical flysch. Penecontemporaneous deformation structures such as load casts, pull-aparts, convolute laminations and slump balls have been reported from these metasediments (N aha 1961; Gaal 1964). Molasse is the product of post-orogenic sedimentation. The deposits, generally several thousand metres thick, consist of many sequences of fining-upward, fluvial and deltaic cycles. The conglomerates contain well-rounded clasts. The fluvial cycles include immature, lithic and feldspathic sandstones, mudstones, coal, caliche, freshwater carbonates and evaporites. In areas of marine transgression the environment may range from paralic to lagoonal. Persistent, well-sorted conglomeratic sandstones dominate the areas of marine transgression. The extensive Cenozoic molasse of the northern Alps consists of clastic wedges, grading upward from autochthonous flysch, through thin-bedded mudstone, to transitional paralic sequences. The succeeding rocks merge to marine deposits laterally because sedimentation kept pace with subsidence to produce near-sea-level surfaces where exchange of marine and non-marine conditions were possible (van Houten 1974). The Miocene-Pleistocene molasse of the Himalayas (the Siwalik Group of India) consists of more than 6000 m of fluviatile conglomerates, sandstones and shales (Figs. 8.2 and 8.3). The palaeodrainage of the Siwaliks was mostly from north to south but northerly drainage systems, controlled by intrabasinal tectonic highs, have also been identified (Tandon 1976, Tandon 1990; Tandon, Kumar and Singh 1985). Confusion has sometimes been created in sedimentological literature by varied· application of the terms flysch and molasse: The terms have been variously used to denote lithofacies, depositional environments, and/or tectonic (syn-or post-tectonic) conditions. The confusion created has prompted some sedimentologists to recommend discontinuation of both terms. It should be remembered, however, that these terms, flysch as well as molasse, have thoroughly pervaded the sedimentological literature and to discard them at this stage would only add to the confusion already created. These terms may thus be retained but the specific sense in which they are used should be fully understood. Modern Geosynclines

Following the classical studies on geosynclines in the late nineteenth and early twentieth century, many a sedimentary basin was identified as a geosyncline. In fact, Marshall Kay, who proposed a comprehensive scheme of classification of geosynclines

Fig 8 2

30° 1'(

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100 km

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The Siwaltk BaSin between the rivers Indus and Ganga (after Tandon 1990) Reprinted wIth permIssIon of the author

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Extra formational Clasts

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I~l Trough Cross-bedding Azimuth

~

Parallel Bedding

D

Massive Sandstone

[2]

-

Planar Cross-beddmg AZImuth Over bank Deposit

Fig. 8.3:The Middle Siwalik lithological columns, Oehra Dun region (after Kumar and Nanda 1989) With permission of the Geological Society of India

212


in 1951, preferred to designate every sedimentary basin located within, outside or adjacent to cratons as a geosyncline. Such a scheme of classification (Table 8.l} apparently accepts a geosyncline as a universal phenomenon, both in time and space. Following the Principle of Unifo!mitarianism, one would, in such a case, expect the geosynclines to be present today as extensively as in the geological past. Qur difficulty in recognizing modern geosynclines, however, has raised doubts about the very basis of the geosynclinal concept. Table B.1: A scheme of classification of geosynclines (simplified after Kay 1951)

1. Orthogeosynclines (linear troughs located between cratons) (a) Eugeosynclines (actively subsiding areas, volcanics present with sediments) (b) Miogeosynclines (less active tectonically, volcanics absent) 2. Parageosync1ines (ovate troughs located within or adjacent to cratons) (a) Exogeosynclines (extensions of orthogeosync1ines) (b) Autogeosync1ines (isolated basins within cratonic uplifts)

During the early years of this century the deep oceanic trenches of Indonesia and the bordering volcanic arcs were intensively sfudied by Dutch geologists and geophysicists following a suggestion from Haug in 1900 that these deep oceanic trenches are the modern counterparts of geosynclines. The concentrically distributed Indonesian Arcs (the Banda and Sunda Arcs) although accepted by many as the basis for explaining the evolution of the East Indies, failed to provide a model comparable to the largescale geosynclines found elsewhere in the world. The sediment layers identified within the Java and Japan Trenches, for example, are comparable neither in thickness nor in character to those characterizing ancient geosynclines. In the Atlantic Ocean, off the North American coast, thick prisms of submarine sediments (named 'continental terrace') have been interpreted as the modern analogues of orthogeosynclines (Drake, Ewing and Sutton 1959). Similar thick sequences of suboceanic sediments have been termed 'continental rise' elsewhere (Dietz 1963). Some of these submarine deposits are enormously thick-the estimated thickness of the Bay of Bengal Deep-Sea Fan, or example, exceeds "12 Ian (Curray and Moore 1971). Many authors would like to associate these deep-sea sediments with the phenomenon of seafloor spreading and would imagine them to be the modern counterparts of classical geosynclines (Dietz 1963; Dietz and Holden 1974). In contradiction to this hypothesis, it has been pointed out that the sediments of the continental rise were derived from the continental side only, while those of the ancient geosynclines were drawn from both the foreland and borderland (Pettijohn 1975). The idea of whole ocean basins being considered modern representatives of geosynclines being dragged towards mobile belts has been questioned by Pettijohn (1975, p. 577) on the ground that in that case the pelagic carbonates would have ended up in deep oceanic eugeosynclinal sequences. There is no evidence, according to Pettijohn, that this has happened. Of late, several workers have reported the presence of CO 2-rich inclusions in granulites (see for example, Hansen, Newton and Janardhan 1984; Newton 1988). It has been suggested that one of the possible sources of CO 2 is the deeply buried carbonate sediments. This leads one to wonder whether the source


213

behind the carbonate fluid was the pelagic carbonate which was actually carried to the mantle by the process of subduction. PLATE TECTONICS CONCEPT

A model of global tectonics, called the plate tectonics model, emerged during the two decades of intensive exploration in the sea following ;he Second World War. During this period palaeomagnetic studies provided evidence of secular changes in palaeolatitudes, thereby confirming large-scale movements of the continents. Detection of n~wer oceanic crusts with 'magnetic stripes' being added to the ocean-floor on either side of the midoceanic ridges since the Mesozoic, confirmed the idea of sea-floor spreading. The major dynamic features of the earth such as orogenesis and seismicity could be connected to the phenomenon of gliding of the rigid lithospheric plates over the asthenosphere. It was also recognized that the lithosphere is consumed at the Benioff subduction zones, on the landward side of the deep-sea trenches, so that the addition of newer crust at the midoceanic ridges does not change the net global surface area. Following these findings six major and a few minor lithospheric plates were identified and the plate boundaries were divided into three groups, depending on the nature of their movements (McKenzie and Parker 1967; La Pichon et al. 1973). These are: (I) Diverging pIa te boundaries at the midoceanic ridges-accreting margins where new oceanic crusts are added to the ocean-floor. (2) Converging plate boundaries (called subduction or Benioff zones) at the submarine trenches defined by the plane where the deep foci earthquakes are located. The lithosphere disappears below the mantle at these boundaries. (3) Transform fault or shear boundaries where the plates glide past one another without addition or destruction of the crust. , The continental margins, on the basis of their coincidence (or not) with the plate boundaries, are divided into two groups, 'active' and 'passive'. The western margin of the continent of South America, which is actively involved in plate movement, is an example of an 'active' margin, while the eastern margins of North and South America, not coinciding with any plate boundary, are 'passive' margins (Fig. 8.4). The setting of a sedimentary basin can now be described in terms of three factors: 1) the substratum of the basin, 2) proximity of the basin to the plate margin and 3) the type of plate margin involved in basin development (Dickinson 1974). In this model the continental and the oceanic crusts are the two end-members of a series of' transitional types of substrata. In terms of proximity to plate margin, Dickinson (1974) divided sedimentary basins into two groups: 1) those located within the plates ('intraplate basins') and 2) those lying at the zones of plate interaction. Three different types of plate junctures described earlier, namely, divergent, convergent and transform, were also recognized. For each of the four broad classes of plate settings (intraplate, divergent, convergent and transform), Dickinson recognized four subclasses based on the nature of substratum. Dickinson's model provided a basis for classification of sedimentary basins from the newer perspective of plate tectonics, but the real situation is even more complex than that stated above. Because of the movements involved over long spans of time, anomalous crusts, inherited from the previous plate junctures, may be incorporated into the substratum of intraplate basins. Furthermore, substrata of both continental and oceanic types may be present in the basins located on the zones of divergent,

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215

convergent or transform plate boundanes For these reasons a drrect correlatIOn between the sedImentary basms classIfied accordmg to the geosynclinal and plate tectOnIC models IS not pOSSIble As Readmg (1982) very aptly commented 'The old geosync1mal termmology can now usually be abandoned proVIded It IS realIzed that many sedImentary basms fall mto more than one category and that lllterpretations of the anCIent are never more than workmg hypotheses'

PLATE MOVEMENTS AND BASIN FORMATION The mechanism of ongm and evolutIOn of large sedimentary baSinS can now be discussed In terms of dlvergmg (extensIOnal), converglrtg (compresslOnal) and strzke-sllp plate movements Some of the current models of baSin development under these SItuations are bnefly presented below The readers may refer to Readmg (1986) and Shannon and Naylor (1989) for more detailed diSCUSSions The models discussed here are based largely on theoretical studies and partly on actual geological-geophysical observatIOns on basm architecture, lIthiC fill and lIthosphenc conditIons They should be taken only as tentatIve suggestIons Not one IS fully proven Basins in Extensional Setting

The Simplest type of extenSIOnal model envisages upwellmg of the asthenosphere dunng passage of the contlllents over hot spots leadlllg to domlllg of the lIthosphere Subsequent COOllllg of the crust and erOSlOn of the dome leads to generatIOn of sedimentary baSinS (FIg 8 5) In another model thmnmg of the contmental crust by rapId stretchmg of the lIthosphere IS contemplated to Imtlate subSidence and thermal anomaly Slow coolmg, followmg WIthdrawal of stretchmg, leads to thermal contractIon and subSIdence (McKenZIe 1978, JarVIS and McKenZIe 1980) SeismIC eVidence of thInmng of the crustal layer below the North Sea proVides support for thIS model The phases of crustal subSidence and manne onlap worked out III the North Sea on the baSIS of thIS model match actual observations At a later stage, Isostatic adjustments may mtroduce comphcatlOns to the basms developed m thIS way More complex, fault-controlled models for basm development have also been postulated These models are based on detectIOn m the Rocky Mountallls and the Colorado Plateau of major zones of shear traversmg the whole Itthosphere (Fig 86) These lllclude the umform Simple shear, non-umform pure shear and mixed shear models A shear traversmg the lIthosphere offsets the reglOns of maXImum thlllmng of the crust and the mantle The basms developed on such a shear are naturally asymmetnc III shape Moreover, complIcatIons may anse due to differential response of the upper bnttle crust and the lower ductIle mantle to shear movements (WernICke 1981) The initial site of nftmg IS controlled to some extent by the composltlon of the crust and the mantle The vertical strength profile of the ltthosphere IS mfluenced by these compOSitions For a dry, granulItiC lIthosphere, the strength profile mcreases downwards, through the crust, only to decrease sharply at the ductile mantle Th IS makes It difficult for the upper bnttle sectIOn to stretch The presence of water pushes up the ductIle level, thereby makmg stretching of the hthosphere easy


216

- Eroded Dome

-- - ---

I,

, 1-,, . . . . -_''',

-.

-"

Sedimentary Basin

, ,-=-,

,, ,,

~ Sediments ~

Continental Crust

(. . ::1

---~ - Mantle Lithosphere

--

Hot Spot

Fig. 8.5: Mechanism of development of sedimentary basins in response to thermal doming (after Shannon and Naylor 1989). With permission of Graham & Trotman, . :-.:-: .: '. s"{ riI-~'E-~ j ,. ........, '

_

'"""

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BASEMENT CONTINENTAL

.... :

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MAN1LE ,,~~ LITHOSPHERE ~:" ~

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'...:""f -; ;- " :, -

_ _ _ _ _ MANTLE _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ .Jf LITHOSPHERE

Fig. 8.6: Diagrams illustrating the mechanism of development of 'thin-skinned thrusting' in areas of compression (after Shannon and Naylor 1989), With permission of Graham & Trotman.


217

The processes responsible for formation of grabens within the continental crust and also for continental rupture creating new sea-floor are similar in many ways. Both are associated with tensile f«ulting, crustal thinning, warming of the mantle below the rift zone and up doming of the outer flank of rifts. Thermally controlled subsidence occurs as the anomalously warm mantle below the rift cools. Lateral heat flow may cause transient uplift of the graben flanks (Jarvis 1984). The uplifted basin flanks, in turn, provide sediments to the basin. In another model of basin development by extensional process, three-armed, radial rift systems ('triple junctions') are contemplated to be produced by convective currents originating in the mantle. Large ocean basins are believed to be produced by spreading of the two active rift arms, while re-entrance of the third arm into continental platforms initiates rift basins. These abandoned rift basins have been termed' aulacogens' (meaning 'born as furrows') by Russian geologists. The tectonic movements in aulacogens are vertical and the sediment transport direction is mainly longitudinal, along the basin axis. Alkalic (basalt-rhyolite) rather than tholeiitic volcanic activity is another characteristic of the aulacogens (Hoffman et at. 1974). The Benue Trough is generally cited as a classical example of a rift arm abandoned during separation of Africa and South America during the Cretaceous (Fig. 8.7).

oo------~~~~~--------~------

Cretaceous

Eocene

Recent

Fig. 8.7: Maps showing $tages of origin of Benue trough as a rift arm abandoned dUring separation of Afnca from South America (after Hoffman et al. 1974) With permission of SEPM (Society for Sedimentary Geology).


218

Basins in Compressional Setting Seismically active subduction zones mark the converging plate margins. On the landward side of these subduction zones, which dip towards the continents, lie volcanic island ·arcs. On the seaward side occur deep-sea trenches. Orogenic belts are produced out of collision of the lithospheric plates. Intrusion of the subducted material into the lower crust causes doming of the ign,eous and metamorphic terranes at the zone of convergence, into what are called 'arc massifS'. These elevated areas at the basin margins serve as the source of sediments while the sediments themselves are deposited in one of the following five areas: within the ocean, on the down-going oceanic plate, within the deep-sea trench, in small basins on the slope of the inner wall of the trench, seaward of the volcanic arc (outer arc or for~arc basin), and behind the volcanic arc (back-arc basins). Thick sedimentary piles are stacked in the form of accretionary prisms in the foreares. As the subduction zone migrates towards the sea the forearc sediments are successively thrust upwards (Fig. 8.8). Small sedimentary basins may develop over these accretionary prisms. Those lying between the volcanic arc and the trench slope break are generally very wide (50-100 km). These are the forearc or outer-arc basins. Behind the volcanic arc occur the back-arc basins. ~

Frontal ridge/

+ Accretionary prism ~ Trench Outer swell/

Fig. 8.8: Model of an obliquely subduCting margin (complied by Reading after Walcott 1978, and Lewis 1980, reprinted from Reading 1982 with permission).

The sediments deposited within the accretionary prisms undergo intense tectonic deformation during movements of the subduction zones. Seismic proftIes run across many of these deformed areas, such as the Rocky Mountain Province, showed that most the structural complexities are restricted to ,the uppermost part of the crust (approximately the upper 10 km) where it is most brittle. These complex structures are replaced underneath by only a few simpler, poorly defmed thrusts going down almost to the base of the lithosphere. This phenomenon has been termed 'thin-skinned compressional tectonics'.

219


Basins in Strike-slip and Transform Fault Zones Strike-slip fauits may occur either at the major plate boundaries or at the border of microplates. Transfann faults occur as fracture zones running across the midoceanic ridges. The overall movement in a strike-slip fault is essentially horizontal and parallel to the fault trace but the dip-slip component may also be important locally. This oblique movement causes the strike-slip motion to be either divergent (transtensile) or convergent (transpressive). The former is likely to produce normal faulting, leading to basin development and volcanism. The latter produces reverse faults, thrusts, uplift and folding. With the passage of time, a divergent strike-slip fault may be converted to a convergent one. Curving, splitting and offsetting of strike-slip fault trends may lead to local zones of extension and compression. The latter produces sedimentary basins while the former leads to uplifted areas which eventually serve as the source of sediments (Reading 1982; also see Fig. 8.9). Upthrust block

Antithetic strike-slip fault

Extension & subsidence Fig. 8.9: Diagram illustrating the mechanism of basin development from dextral strikeslip fault. Both extensional basins and compressional uplifted areas serving as source of sediments may be produced as a result of such faulting. Direction of sediment transport is indicated by thick arrows (after Wilcox et al. 1973. modified by Mitchell and Reading 1978. reprinted from Reading 1982 with permission).

TECTONIC CONTROL OF SANDSTONE COMPOSITION The technique of interpretation of tectonics from sandstone mineralogy and chemistry is based on the belief that the detrital framework mineralogy of a sandstone is dependent largely on the tectonic setting of its provenance. This technique of interpretation should be used with caution, however, because the mineralogy of a sandstone may be considerably modified by sorting processes during transportation. Moreover, diagnostic processes may also obliterate the original detrital mineralogy and chemistry of a sandstone.


220

Quartzose sands from continental cratons are widespread within interior basins, platform successions and opening ocean basins. Arkosic sands from uplifted basement blocks are present in rift troughs and wrench basins related to transform faults. Volcaniclastic lithic sands and volcanic-plutonic sands derived from magmatic arcs are found in trenches, forearc basins and marginal seas. Recycled orogenic sands, rich in quartz or chert and other lithic fragments, occur in closing ocean basins, diverse successor basins and foreland basins. These sands are derived from subduction complexes, collision orogens and foreland uplifts. Using triangular QFL and QmFL plots of quartz (mono- apd polycrystalline varieties), feldspar (both plagioclase and K-feldspar) and lithic fragments (volcanic/metavolcanic and sedimentary I metasedimentary), Dickinson and Suczek (1979) could discriminate these tectonic settings (Fig. 8.10). Bhatia (1983) could discriminate the Palaeozoic sandstones from various tectonic settings of eastern Australia with the help of their major element geochemistry. Plots of Fe20 3 + MgO versus Ti0 2, A1 20/Si02: K 2 0/Na2 0, and A12 0 2 /(CaO + Na 20) were utilized for this purpose. The oceanic island arc sandstones, derived from calc alkaline andesites, are characterized by higher proportions of Ti0 2, A1 20 3, Na 20, and Fe 20 3 , and a relatively lower proportion of SiOiand K 2 0 than all other sandstones. The continental island ·arc sandstones, derived from felsic, v-olcanic rocks, are characterized by higher proportions of Si02 , K 20 and the K2 0/N a20 ratio (:::;0.6) and lower Fe Z0 3 + MgO than the oceanic island arc sandstones. The sandstones from the active continental margins (Andean-type and strike-slip basins) are derived mainly from the uplifted basement. These sandstones, reflecting the composition of the upper continental crust, have high Si0 2 and KzO content with K 20/Na20 ~l. The passive margin sandstones are generally enriched in Si02 and depleted in Al 2 0 3 , Ti0 2 , Na 2 0 and CaQ. The K 20/Na 2 0 ratio of these sandstones is more than 1, but these sandstones may show -large variations in composition, sometimes even overlapping the compositions of active continental margin sandstones (Table 8.2). Table 8.2: Abundances and proportions of various oxides in sands of various tectonic settings (after Bhatia 1983)

Oceanic island arc (OIA)

Fep~t)+ MgO

8-14 (11.73)

Ti0 2

0.8-1.4 (1.06) 0.24-0.33 AIP3/ Si02 (0.29) K 20/Nap 0.2-0A (0.39) AI203/(CaO + Nap) (1.72) (58.83) SiOz

Continental island arc (CIA) 5-8 (6.79) 0.5-0.7 (0.64)

0.15-0.22 (0.20) 0.4-0.8

continental

Passive margin

margin (ACM)

(PM)

Active

2-5 (4.63) 0.25-0.45 (OA6) (0.18)

(2.89)*

Depleted

(0.99)

(1.6)

(2.56) (73.86)

(4.15) (81.95) Enriched

(0.10)

(0.61)

(2.42) (70.69)

* figures in parentheses indicate the average values.

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222

In another study the framework mineralogy and volatile-free chemistry of graywackes were related by Crook (1974) to specific tectonic settings. Using data from deep-sea sands, taken to be the modern counterparts of arenites in ancient flysch, and sands from rivers debouching into the sea, Crook demonstrated that the quartz-poor graywackes (quartz < 15%, average Si02 = 58%, K 20/Na20 « 1) are indicative of magmatic island arcs. The average composition of these rocks approximate the average composition of these arcs- and that of tholeiitic andesites. Quartz intermediate graywackes (quartz = 15-65%, average Si0 2 = 68-74%, K 20/Na 20 < 1) are indicative of Andean-type continental margins and have approximately the same composition as the upper level of the continental crust. Quartz-rich graywackes (quartz > 65%, average Si0 2 = 89%, K 2 0/Na 20 > 1) are characteristic of Atlantic-type continental margins and are similar in composition to the sand fraction of the continental platform cover. Quartz content (19% average) and K 10/Na 20 ratio « 1) of representative greywacke samples from the Chitradurga Schist Belt (> 2600 m.y_) of India (Tables 8.3 and 8.4) indicate that these belong to the quartz-intermediate type of graywacke of Crook (1974) and were, in all probability, derived from the upper level of the continental crust (Srinivasan et aI. 1989). Table 8_3: Modal composition of the Keshavapura Graywackes from Chitradurga Schist Belt (KD1 samples after Srinivasan et al. 1989, Table 1)

7

M(Avg.}

16.7

19.3

18.9

5.5

7.3

9.9

9.2

2.1

1.4

2.0

4.5

4.7

4.3

10.9

8.2

8.0

l.6

1.2

1.0

1.9

2.0

2.7

3.4

1.0

3

3A

4

5

6

18.2

12.9

20.7

25.9

grains

5.9

10.9

15.7

Monocrystalline grains

5.2

1.6

Plagioclase (P)

6.1

14.3

K-Feldspar (F)

4.7

1.9

2.1

Quartz 'Q' as clusters and polycrystalline augens Polycrystalline detrital

Rock fragments, volcanics

+ glass (R)

1.5

Biotite

30.9

6.5

9.0

47.8

36.9

44.1

Chlorite

25.0

8.9

5.5

5.0

9.9

6.3

36.6

34.8

Sericite 1M uscovite Calcite Opaque minerals

1.5

2.9

29.2 6.1 11.9

1.6

3.3

13.6

6.0

4.1

1.9

3.0

1.1

1.0

1.9

The graywackes derived from various tectonic settings can also be distinguished on the basis of their immobile trace element content. From oceanic island arc through continental island arc, active continental margin to passive margin settings in eastern Australia, a systematic increase in the light rare earth elements (La Ce N d) Th Nb and decrease in V and Sc were noted. The ratios of Ba/Sr, RblSr, La/Y, and Ni/Co increase while the ratios of Ba/Rb, KITh, and K/U decrease in the same direction ,

,

,

I

223


(Bhatla and Crook 1986) Usmg a slmllar approach Snmvasan et at (1989) suggested an lsland arc settmg and a mlxed felslc and mafic source (as mdlcated by the hlgh proportIon of Zr as well as Cr and N1, see Table 8 5) for the Keshavapura Graywackes of the Chltradurga Schist Belt of Karnataka, India Dlscnmmatlon between oceamc and contlllentallsland arcs was, however, not posslble Table 8.4: Chemical composition of the Keshavapura Graywackes from Chltradurga Schist Belt (KD1 samples after Srinivasan et al. 1989, Table IIA)

Wt%

3

SI02 T10z AI 20 3 FeO

6105

Fe203 CaO MgO

3A 5805

4

5

6

7

8

61 16

6766

66 70

64 20

5880

064

068

064

046

046

057

080

14 10

1433

1413

1393

1405

1462

1463

644

648

520

336

292

420

7 12

3 01

402

393

394

383

4 04

470

481

471

372

332 424

257

194

6 05

625

455

172

1 42

242

438

NazO

3 05

330

379

270

366

343

321

KzO

1 50

1 15

109

1 75

2 00

2 41

304

003

MnO

008

010

006

003

004

008

PzO s

o 13

013

012

008

008

009

o 10

066

053

o 71

o 71

070

LOI

052

1 19

BASIN CLASSIFICATION AND DESCRIPTION The concept of plate tectomcs prompted classificatIOn of sedimentary basms m terms plate movements (see, for example, Mlall1984, de V Klem 1987) The comprehensive scheme of classlficatIOn of Shannon and Naylor (1989) reproduced m Table 8 6 relates sedimentary basllls to crust type, nature of plate movement, dommant structural movement and locatIOn of the basm With respect to the plates The characters of some of the major basm types are dlscussed below wlth reference to thiS scheme of claSslficatlOn, drawmg examples from India (Flg 8 11) and elsewhere Many of these groupmgs, based on scant mformatIOn, are necessanly tentatIve Many of the older sedimentary basllls are polygenetlc because they passed through several stages of development at different times of their eXlstence Downwarp Basins

These large, elongate basms develop by downwarpmg (saggmg) of the cratomc shelves bordenng oceans In these basms sheets of stable shelf quartz aremtes and carbonates overhe deeper water argillaceous sediments The West Canada Baslll of North Amenca, the Saudt Arabtan Platform of the Middle East, and the Vmdhyan Basm of central India are some of the typical examples of downwarp basms The Canadian Shield slopes smoothly towards the west unttllt reaches the eastern boundary of the Rocky Mountam thrust belt ThiS downwarpmg, which took place III the Late ProterozOIC, accommodated sediments rangmg III age from the CambrIan to


224

the Lower Jurassic. The deposits, which wedge out on the Canadian Shield, attain a maximum thickness of about 6 km on the western side bordering the Rockies. Marine and nearshore sediments, ranging in age from the Cambrian to the Lower Jurassic, were initially deposited on this downwarped cratonic shelf. These were followed I;>y the deposition of thick marine and non-marine sediments during the M;iddle Jurassic to the Lower Tertiary on a foreland basin which developed east of the orogenic belt. The whole sequence, from the Cambrian to the Lower Tertiary, is punctuated by four major unconformities caused by uplifts related to westward movement of the North American plate. The sedimentary facies and structural elements of the West Canada Basin were controlled by these tectonic pulses (Shannon and Naylor 1989). Table 8.5: Trace and rare earth element composition of Keshavapura Graywackes from Chitradurga Schist Belt (KD1 samples after Srinivasan et a/. 1989, table liB)

(ppm)

3

JA

4

.5

6

7

Zr

141

153

172

221

228

217

Y

32 395

24

27

44

42

45

44

Sr

382

467

687

466

288

Rb

53

40

41

65

70

378 . 76

327

389

461

448

599

V

406 140

293 147

121

76

106

155

Cr

423

414

280

122

161

193

Ni

69 47

50 48

56

Cu Zn

80 68

105

106

102

72 114 34 55 91

Ba

8 188

82

24

24

67

51

51

84

103

108

La

23.40

52.14

33.65

Ce Pr

51.18

81.77

67.34

5.8

8.17

6.95

Nd

21.62

26.42

Sm

4.16

Eu

0.76

30.83 4.79 0.78

Gd

4.72

5.11

4.56

1b

0.66

0.65

0.64

Dy

2.94

3.12

3.31

1.70 < 0.03

1.54 0.30

1.27 0.24

Yb

1.12

1.40

1.03

Lu

0.22 217

0.30

0.24

Br

Tm

KIRb

232

235

220

234

4.84 0.76

259

304

Rb/Sr

0.13

0.10

0.08

0.09

0.15

0.20

0.28

Ba/Sr

7.66

7.32

7.97

5.98

6.58

5.89

7.30

Zr/Y V/Ni Cr/Ni

4.4

6.37

6.37

5.02

5.42

4.82

4.27

2.02

1.83

2.42

2.10

3.16

4.40

2.76

6.13

5.17

5.60

3.30

5.08

6.70

3.44

Plate Location

Movement

Dominant Plate

Crust Type

W Canada Vindhyan

Downwarp

1

Sag

:I

MIChigan Jalsalrruf Blkamr-Nagaur

Aden

E.Afnca

Jhana Benue Cud~pah

Ganga Valley

West Canada

Foreland

ng

Bengal Basrn

Multiple History (CompOSIte) ... ChmaBasm

Assam-Arakan

L:

Interior

I

Compres~lOn

Compres~ion

:

~

Gabon Kutch·Cambay Narmada Andaman

Pull·apart

1

IntcnoT

MMgm

II Dlvrnt.

Niger

Delta

sag'Tting

Faulting Sediment Loading

~r

Divr~t

t I

Caltforma

Subduction

ng

MIn

Mlgm

I conrrnt conrrnt

I

Oceanic to Transitional Crust

1 wrer wrer

Sag

Rift

1

,L

Faulting

~

~

I

Continental Crust

Table 8.6: Basin classification based upon genesis and tectonic location (after Shannon and Naylor 1989). with examples of basins inserted.

...

o

(J\

N

I\)

::l

o

::!:

I I)

::l

CD

-

3·

a.

Co (f) CD

::l

II)

(/)

o·

::l

o

';}


226

o

100 200

I....-..I.--J km

70° Fig. 8.11 : Sedimentary basins (stippled) of India (redrawn from Berger et a/ 1983) With permission of Schlumberger Technical Services, Inc

The Vmdhyan Basm of central IndIa, whIch IS morphologIcally sImilar to the West Canada Basm, apparently started as a downwarp on the northern fringe of the IndIan Shield LIke the CanadIan basm the floor of the Vindhyan Basm slopes smoothly to the north unUlit abuts agamst the thrust belt of the Outer HImalayas (Flg 8 12) The exposed part of the Vmdhyan basIn, whIch occupIes an area of more than 120,000 sq km IS composed of 5250 m of Precambrian (rangmg to early Cambnan) sedIments III the thIckest part Another 1000 m of sedIments are belIeved to be conc~aled below an area of about 40,000 sqkm m the Ganga Valley (FIg 812). The crustal warpmg leadmg to the formatIon of thIS bas III followed the ProteroZOIC DelhI Orogeny. Followmg encroachment of the sea from the north, the Vllldhyan Basm enjoyed beach, shallow manne, barner bar, tidal flat and locally, lagoonal environments at


227

various times. An essentially quartz arenite-carbonate type of sediment (with minor amounts of shale, chert and pyroclastics) is found in the lower part of the sequence. In the upper part a much larger proportion of shale occurs, together with some limestone and quartz wackes. The lower sequence, which is separated from the upper one by an unconformity, is structurally more deformed than the upper one. An Archaean granitic massif (the Bundelkhand granite) separates the exposed part of the Vndhayn Basin into the eastern and the western sub-basins (Fig. 8.12). The Bundelkhand massif extends to the north, below the Gangetic alluvium, almost up to the Himalayan foothills. Geophysical surveys have indicated the presence of a number of basement faults subdividing the Ganga Basin into several horsts and grabens (N arain and KaiIa 1982). The major palaeocurrent pattern was northerly or northwesterly during most of the time. The sediments in the eastern part of the basin were supplied by the older Bijawar rocks bordering the southern fringe of the basin (Banerjee and Sengupta 1963; Banerjee 1974; Srivastava et al. 1983). . Following collision of the Indian and the Eurasian plates, the present day Ganga valley was converted into a foreland depression and the northern part of the Vindhyan sediments was covered by predominantly non-marine, Upper Miocene to Pleistocene Siwalik sediments derived from the newly developed Himalayan orogenic belt.

Rift Basins The Red Sea, and the Gulf of Aden are examples of rift basins formed during early stages of sea-floor spreading. The cause of rifting in these areas was lithospheric extension accompanying thermal doming. The rifted blocks were faulted and titled landwards as the Red Sea Basin began to form during the Tertiary. Basalt erupting through the fissures as a result of mantle upwelling floored the Red Sea and Danakil Depressions. About 3000 m of volcanics and volcaniclastic rocks have accumulated in these depressions. These pass laterally into marine pyroclastics and evaporites (Reading 1982; see Fig. 8.13). The East African rift system, the most important of all the continental rift systems, extends for a length of about 3000 m and is about 40-50 km wide (Fig. 8.14). The rift, flanked by steep escarpments, is often broken and off-set by cross-fautls, volcanoes and horst blocks rising well above the plateau. The drainage flows mostly away from the rift valley, keeping the valley starved of sediments. Only small quantities of clastic sediments, derived from the fault scarps and uplifted blocks, are transported by streams along the rift. Alluvial fans and.1akes constitute the main areas of sedimentation. A number of lakes (e.g., Lake Albert, Lake Tanganyika) occur along the rift. These are mostly freshwater lakes but alkaline-saline lakes, whose chemistry is controlled by the nearby carbonatite volcanics, also occur along the rift (Reading 1986). The Permo-Triassic Gondwana basins occupying three well-defined belts in the eastern part of the Indian Shield may be cited as Indian examples of intracontinental rift basins. The most well-studied of these Gondwana basins, the Jharia Basin in eastern India, was initiated by a general downwarping of the basin floor. At a later phase, the basin was intersected by a large number of parallel faults reaching the basement as a result of crustal tension. The smaller subbasins later merged into a single major depression, which subsided as a narrow intrabasinal graben bounded by contemporaneous normal faults. Sinking of the inselbergs along with rising of a basement ridge, reactivation of old faults and development of new post-depositional

I

,.")

,

,

.~

I

,

~

,,

\

,

...,

,

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. . \<'\! e \./'\.p'-

,..

\

,

r +

t

t

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o,

SO , Scale

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P

,

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Ii'

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-"Allahabad'

,-'

,,

+

"\

<'1

\" .

, <.

IJ

(-

' ....

'

.

Patna

t, _

... /'\

Fig. 6.12; (A) Exposed areas of the Yindhyan Basin (B) Isopach map of the Vindhyan deposits in the subsurface of the Ganga plains based on seismic-cum-geological information, Contours indicate thickness of the Vindhyan sediments In metres (generalized and redrawn after Narain and Kaila 1982), With permission of H, Narain,

"

)

"-

(

@

N ...,

5"

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\C

0"

o

:J

II)

3"

..

a.

en n>

o

::l

o

s:::

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aa.

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CD

Escarpment

1000

2000

km

50

Red Sea

1

Volcanics Oligo-Miocene Marine Pyroclastics Evaporites

.1 Coastal plain I

------_ ..

EAST

1970, reprinted from Reading 1982 with permission).

Fig. 8.13: Section across the Red Sea and Danakil depression (modified by Reading after Hutchinson and Engels

o

·m

Pliocene-Recent Evaporites

1

Danakil Alps

Ethiopian PlaIeau 1 _lDanakil D~Fression

WEST

~

CD

:J

~ o

CD ::J

...

3'

a.

CD

en

0-

:J

Q)

(JI

n'

::J

~ o

n


230

\-

t

L TanganYIka

10'

o l

km I

300 I

Fig. 8.14: Map of the East African Rift System (modified by Reading after King 197 reprinted from Reading 1982 with permission).


231

faults caused the basement to rise at either ends of the basin, thereby providing a source of sediments. Faulting, caused by crustal tension, continued until the deposition of the youngest sediments within the basin (Fig. 8.15). Sedimentation within this fault-bounded graben took place by fluvial and fluviolacustrine processes. At a later stage the fluviatile clastics were intruded by dykes and sills of dolerites and lamprophyres. Emplacement of the lamprophyres, which are restricted to the basinal areas only, post-dates deposition of the major part of the Gondwana sediments. The tensional cracks, active since the early phase of basin formation, reached the mantle at an advanced stage of basin development, causing release of pressure, lowering of temperature and consequent intrusion of lamprophyre dykes in zones of maximum subsidence (Ghosh and Mukhopadhyay 1985).

Failed Rifts and Aulacogens The 1000 km long and 100 km wide Benue Trough in the Gulf of Guinea in West Africa is generally cited as the best example of an aulacogen formed during drifting of Africa and South America. This trough is filled by more than 5 km of fluvial, deltaic and marine Cretaceous sediments. These are overlain by another sequence of fluvial, deltaic and submarine fan deposits extending into the Atlantic Ocean (Fig. 8.16). The Cretaceous aulacogen phase was separated from the Post-Cretaceous sediments by a phase of folding,' followed by erosion (Reading 1986). Interior Basins

Interior basins occupy large circular or elliptical areas within cratons. They are generally symmetrical in profile. Their lithic fill includes clastics and carbonates. The Michigan Basin, located in the east central part of the United States, is an example of an interior basin. This basin was initiated by thermal events. Deep crustal metamorphism leading to phase transformation (gabbro to eclogite) might have led to subsidence. This was followed by thermal sag throughout the Lower Palaeozoic in a setting of low geothermal gradient. The sagging was effected by periodic movements along basement faults throughout the Palaeozoic. The basin contains clastic and nonelastic sediments ranging in age from Precambrian to Jurassic (Shannon and Naylor 1989). The Jaisalmer and Bikaner-Nagaur Basins of Rajasthan, India are interior basins. Bikaner-Nagpur, which was initially a stable shelf up to the Early Palaeozoic, later developed into a restricted ('silled') basin to accommodate about 2 km of Cambrian~ Triassic sediments (Berger et al. 1983; see Fig. 8.11). The Cuddapah Basin occupying a crescentic area of about 42,000 sq km in the south-eastern part of the Indian Shield is another example of an interior basin. A maximum thickness of about 10 km of clastic and non-clastic Proterozoic sediments is estimated to have been deposited in this basin. The total load of accumulated sediments and emplaced igneous material in thi,s basin cannot account for a subsidence of 10 km. The main driving force for generation of this basin seems to be thermal heating and cooling, followed by isostatic loading. Three major unconformities within the sedimentary sequence of this basin indicate crustal up arching as a result of isostatic adjustment to rising temperature in the lithosphere. Rising temperature not only caused thermal expansion but also reduced the density and thickness of the lithosphere, A maximum crustal thickness of about 38 km below the basin, with a thinning of about 4 km towards the eastern fringe, is suggested by gravity data.

0

lJ

"'("6::---"

t'

&/

l}

qlp

lJl"p

°6 <1 ~ .;~

~ ~ F ~d I :La,."u"""""

•

Jharia

Fig. 8.15: Contemporaneous faults of the Jharia Basin. India (after Ghosh and Mukhopadhyay 1985). With permission of Geological. Mining & Metallurgical Society of India.

Barren Measure and Raniganj 0 oD oQ

Upper Barakar, Barren Measure and Raniganj 0 x:::J xD

Middle Barakar II and Upper Barakar------Upper Barakar and Barren Measure eeee Middle and Upper Barakar and Barren Measure BEtflB

: Barren Measure 0 G0 Fault active in Middle Barakar I and Upper Barakar xxx

: Upper Barakar ----------

.... 0

~ '"to ....to

"r

J a r

1>..........;:::

~

/;>,

+~>C >c Ja'Q",Q'

~

lJ 00 00 t OO~l"'''1 d "'0 0

0

q

urnarda fault

vI",

u.r i

+

OOoooo~

F

00 oooa lJ 1 r

a bad

° ;. .

qp

<>

000 Fq

VI

~ Ie -\p~

..,.. .... \.. e

Fault initiated and active only in: Middle Barakar 1 +++ : Middle Barakar II nn"n

00 Co

FaUlt

H

0

-0" r h 000 'n 0

sO 0 0

°0 0Q

Barnangora ~ 000

Contemporaneous faults

'-

0

'Q

0 0000 000

+QOOOOOOooo flariharpur

)()()(

-J."f-

~~~~~~ .....

~

'<

co

0-

0"

::s

CD

3

~

(f)

6"

c)" :l

~

t:

Q.

a

;-

N Co) N

I

rae::,'

7

NIger delta M!Ogeoclme-----~> <

.'. DELTA

Benue trough

~

km

fudge

I

250

NE =>Wi

Basin

)~Zambuk~~Bornu_7

Fig. 8.16: Cross-section through Benue 'FaIled RIft' (aulacogen) and the TertIary NIger Delta (modIfIed by Reading after Burke et al 1972, Petters 1978, reprrnted from Readmg 1982 WIth permIssIon)

10

5

< SW Or

N

'"w

::J

~ o

::J

(I)

-

3'

0-

en (I)

c-

::J

II)

Ul

(=r

::J

~ o

o

234


Step faults, tlltmg and local vanatIOns m the depth of the basm-floor revealed by Deep SeIsmIc Soundmg (DSS) profiles run across the Cuddapah Basm were posslbly caused by flexuralloadmg. These deep-seated faults also provlded passage for upward mlgration of mantle flUIds (kImberhte-carbonatttes) and other partial melt products, namely, calc alkalme, alkalme and felsIc magmas That the role of tensIOnal faultmg was dimimshmg dunng the later phase of deposltlOnal hIstory lS shown by dwmdlmg of Igneous aCtIvIty after lower Cuddapah sedimentatlOn CompresslOnal rather than tenslOnal forces were actIve dunng the later (and post-depoSltional) phases of basm evolutIon. ThIS is indicated by the presence of thrust faults in the eastern margm of the basm (BhattacharJi and Smgh 1984) Foreland Basins Foreland basins generate at the toe of the overridmg plates In the areas of continentocean or continent-contment colhslOn. SubsIdence In the foreland basm IS caused largely by subductIon The lithosphere m front of the overrIding plate IS depressed also by the weIght of the thrust shces As the basin subslded, a relatIvely low but wIde bulge developed on ItS cratomc slde The older surface, where not covered by deep water, IS eroded m the form of an unconformity The western part of the West Canada Basm consIstmg of a folded and thrust belt on the western margIn and a broad un deformed platform on the eastern sIde is descnbed as a foreland basm (Cant 1989) The basm is elongate parallel to the orogemc belt The preserved Mesozolc rocks occupy the southern part of thIS basIn while PalaeozOIc carbonates occupy the northern part. The basm continues to the south to merge wIth the Western Intenor Basm of the United States Reverse faults Involving the basement diVIded the foreland basm m the latter area mto a senes of smaller basins dunng the Late Cretaceous Laramide Orogeny. The West Canada Basm Imtially developed as a downwarp to accommodate sedIments ranging m age from the Cambrian to the Lower Jurassic Apparently as a result of the LaramIde orogeny, the same basin was converted to a foreland basin and recelved sediments rangmg in age from the Mlddle Jurasslc to the Lower Tertiary. Farther south, in South Amenca, the LaramIde basins in the foreland of the Andes resulted from shallow dippmg subductIon, causing drag at the base of the lIthosphere (Cant 1989). A sImilar pIcture of basm development can be traced III the Ganga Valley, IndIa, where the downwarped ProterozOIC Vindhyan Basill ill the northern part of the IndIan Shleld gave way to a foreland basill, followmg the Himalayan orogeny, to accommodate SIwahk sedlments of MlOcene-Pleistocene age. The Assam-Arakan Basm located between two thrust belts m north-eastern Indla IS an example of a foreland basin. The TertIary sedIments of the Upper Assam belong to two dIstmct deposItional facles - a relatIvely shallow-water shelf facIes and a deepwater geosynchna1 facIes Between the Himalayan and the N aga-Disang thrust complex occurs an autochthonous zone ('the foreland spur'; Fig 817) contaming sediments rangmg m age from Eocene to PleIstocene These are intersected by a number of gravIty faults Farther east, ill the Patkal Range, TertIary sedimentatlOn took place under deeper water condItIOns, punctuated by slIght emergence at some places The sea receded southwards towards the end of the Oligocene Miocene sedlmentatIon took place mostly under freshwater to brackish condItions Oligo-Miocene was also the time when the depOSIts of the Assam-Arakan Basm were overthrust towards the north-west, over the

235


north-eastern extension of the Indian Shield. The outermost of these thrusts, the N aga Thrust Belt, consists of a succession of six thrust sheets. In the thrust strip nearest to the foreland spur there are anticlinal structures, such as the one on which the Digboi Oilfield is located (Evans 1932; Berger et aI. 1983).

~~..2;....-----_t--26°

26°------, ~/o. , Vo

-

,

I

,;

:

,--

Manipur

/

Fault .................... . Main synclinal axes .. . Generalized basement Contours in metres Direction of Thrusting

--

~

,

.. ..' ~

x

y

Foothills of Eastern Himalaya

. I

--

.,,--- --- - ~- -- -~~ --_ .. ,'"

Pliocene

.~::.:-:.-:

V

1

Brahmaputra Valley

Probable Position of Outer Thrust·Fault

..--{

Gondwand

---r=~I,

MIocene

Nahorkauya Foothills of 011 field Naga Thrust_Naga hills

Alluvl\!.m

I--- ... ---

liI-

-- -

Plwxent.

-----

Basement

~~A.t,

h.1.1OLLne

.

0L _ _5L.----"lOkm . I ,

Simplified Section Across Upper Assam along

®

..... ~{'1)1.."

---X- Y

Fig. 8.17: (A) Foreland spur of north-eastern India, (B) NW-SE cross-section across the foreland spur shown in (A). (Redrawn from compilations by Berger et al. 1983). With permission of Schlumberger Technical Serlvices, Inc.

236


Subduction Basins The California Basin located at the Junction of the North American continental plate and the Pacific oceanic plate provides a well-documented example of a subduction basin which experienced compressive as well as wrench movements. Mesozoic sedimentation in California took place in forearc basins related to· subduction movements, to be followed by sinistral, strike-slip basin development during the Neogene. The oceanic-continental plate collision and changing direction of plate movement brought complications to the area. In complexity of ehara.cter the California Basin is comparable to the deep-sea basin which runs for a length of about 3000 km from the offshore of Sumatra to the mainland of Burma via the western Andaman Sea (Fig. 8.18). The forearc basin off Sumatra contains about 4 km of sediments including deep-water turbidities, montmorillonmc clays and volcanic ash. As the Sumatran continental block subsided with time and the subduction zone migrated southwards, a grildual widening of the forearc basin took place. The basin shape is controlled by a number of strike-slip faults. In the Andaman Sea a central volcanic arc separates two troughs:- a forearc basin on the western side and a system of rift valleys on the eastern side. The latter, associated with several strike-slip faults, is the spreading centre of several pull-apart basins (Rodolfo 1969). The total sediment thickness in the eastern Andaman basin is small (1.5 km), indicating that the Irrawaddy delta, the main source of the sediments in this basin, came into existence only in recent years (Reading 1982, 1986). In the mainland of Myanmar (Burma) a volcanic arc separates the forearc from the back-arc. The estimated sediment thickness is about 12 km in the former basin and 17 km in the latter. Both basins are possibly underlain by an oceanic crust. The largest sediment cover on a presumably oceanic crust today is the Bengal DeepSea Fan extending from the mouth of the Ganga-Brahmaputra Delta to the Indian Ocean, covering an area of 3,000,000 km 2 of the Bay of Bengal floor (Fig. 7.10). The average thickness of the fan sediments here is 7.5 km but the maximum estimated thickness is of the order of 12 to 15 km. Sedimentation in this fan has been going on since the beginning of the collision of the Indian and the Eurasian plates at a rate of 210 cm/ 1000 years. The phases of fan sedimentation are correlatable with the phases of upheaval of the Himalayas (Currayand Moore 1971; Moore et al. 1974). Pull-apart Basins Pull-apart basins developed at passive continental margins during rupture of the megacontinents and drifting away of the continents. Although they may have been initiated as rifts, pull-apart basins are distinct in the sense that the sedimentary prisms of these basins prograded towards the ocean basins in the post-rift stage. Clastic sediments were deposited during the rift phase while evaporites, carbonates and turbidites dominated the spreading phase of pull-aparts. The Gabon Basin lying on the western eoast of Africa, between the Niger delta and Angola, is a pull-apart basin (Fig. 8.19). The basin was initiated at the time of opening of the South Atlantic, following migration of South America from Africa. The onshore part of the basin has a maximum width of about 200 km. A Precambrian horst block separates the basin into two parts-a deeper basin opening to the Atlantic in the west, and a shallower, interior eastern basin. The Atlantic basin, an extensive monocline,


237

contains more than 8000 m of sediments ranging in age from the Jurassic to the Miocene. A thick evaporite formatiqn of Aptian age has resulted in structural discordance between the sequences above and below. In the eastern basin rocks ranging in age from the Jurassic to the Middle Cretaceous, up to a maximum thickness of 5000 m, occupy a faulted syncline.

EASTERN BURMA HIGHLANDS

IRRAWADDY DELTA ....A-

Thrust fault Strike slip fault

..

Spreadmg ndge Volcano

GULF OF THAILAND

INDIAN OCEAN

Fig. 8.18: Arc-trench system of Burma-Andaman-Indonesla shOWing stnke-sllp faults. volcanic arc, forearc and backarc areas (complied by Reading 1982 after Curray et al. 1979; Kraig et al. 1980). Reprinted with permission.

25

50

75

100km

"~. - -

~ET' 1

""

.

.

..

8000

6000

"000

2000

~ Basement

0 "j,'HINGE ?,-- ..ZONE::

..

-'-- Soli

.. -

•

~~ .. ~' ..

8A SIN - - - -...

~.

rock

D

+

i

+

+

t

+

+ +

+

t

+

+

1-

+

2qo

+

..

t

o

-'

V

-f>

("'I

+

oIMOCAMErESf'i

+

+ -p

.;\

'V

+

MC$oz-C?IC:-T."lory BaSIns

ATLANTIC OC£A N

+

+

Fig. 8.19: Gabon 'Pull-Apart BaSin' of West Africa, and cross-sections shOWing stages of development of Gabon BaSin (complied from Shannon and Naylor, 1989, Bnnk 1974). With permission of Graham & Trotman and American ASSOCiation of Petroleum Geologists

o

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())


239

The Late Jurassic-Early Cretaceous phase of rifting and basin development initiated by continental drifting, was followed in the Gabon Basin by a long post-rift phase of basin development. The rate of subsidence, which was high during the Jurassic to the Lower Cretaceous, gradually declined during the Upper Cretaceous and the Tertiary. The stages of basin development can be traced in the sections shown in Fig. 8.19. The three hinge lines, each representing a cluster of faults seen in the sections, had major roles to play in the development of the basin (Shannon and Naylor 1989). Three sedimentary basins, at Kutch, Cambay and Narmada, occur in the western continental margin of India. The faults bordering these grabens open seawards (Fig. 8.20). The sediments within these basins, ranging in age from the Upper Bathonian/ Callovian to the Neocomian (? Santonian), are essentially clastic, interspersed by calcareous or marly bands at some places in Kutch. A major marine transgression, followed by regression and deltaic environments with periodic marine influence, are the interpreted depositional environments in Kutch (Biswas 1981; Biswas and Deshpande 1983). These rift basins are supposed to have opened up successively as a result of the counterclockwise drifting of the Indian craton after its detachment from Gondwanaland in the Late Triassic or the Early Jurassic. Rifting is inferred to have developed from north to south in the following order: Kutch (Early Jurassic), Cambay (Early Cretaceous) and Narmada (Late Cretaceous), (Biswas 1982). These rift-bounded basins opening towards the sea satisfy the characters of the pull-a part basins of Shannon and Naylor (1989).

Delta Type Basin In the Shannon and Naylor (1989) scheme (shown in Table 8.6), deltas have been classified as sedimentary basins. Deltas, as discussed in Chapter 7, represent a particular type of deposit produced by interaction of marine and non-marine agencies. They may develop whenever a sediment-water mixture is funnelled through a channel into a relatively quieter body of water provided that the rate of sediment input is higher than the rate of its dispersal. Deltas may develop in continental, transitional or oceanic crust, and on any type of plate margin, convergent or divergent. Although all deltas of the world have many broad similarities, their characters differ depending on the geological and tectonic setting and local climatic conditions, which. control the rate of sediment supply. In the Niger Delta, cited by Shannon and Naylor (1989) as a typical delta type of sedimentary basin, a mean annual supply of 2 x 10 7 m3 of sediment is maintained through the Niger River and its tributary, the Benue River. These rives flow through rift valleys believed to be the 'failed arm' (aulacogen) which extended into the continental interior from the 'tripple junction' formed during separation of South America from Africa. The rifting, initiated during the Late Jurassic, led to deposition of a wedge of sediment along the Benue Trough until the Early-Middle Cretaceous. This was followed by deposition of a coastal prism of sediments during the Tertiary. The depositional centre of this wedge gradually shifted towards the sea as the pre-Tertiary depression was filled. The Oligo-Miocene delta complex, for example, is located wholly offshore on the rapidly subsiding oceanic crust (see Fig. 8.16).


240

7

0

24°

2 20"

16'

rlZIStudy area 700 14°

kmSO I

Scale 0 I

7.

1001an I

0

76°

~,

( <... ...-r~

Precambrian tectonic trend Rift faults (with downthrow) Major structural trend within rift system

Fig. 8.20: Kutch, Narmada and Cambay rift basins on the western coast of India (modified after Biswas 1982). With permission of American Association of Petroleum Geologists.

Composite Basins with Multiple History The practice of compartmentalizing sedimentary basins to specific associations of crust type and plate movement, followed in Table 8.6, is artificial to some extent. Many sedimentary basins have passed through a variety of lithospheric movements during their span of existence and hence are polygenetic in character. The Benue Basin, for example, developed into a delta, starting from an aulacogen. The West Canada Basin and the Vindhyan Basin, both of which were initiated as downwarps, experienced foreland depositional conditions following orogenic movements in the bordering areas. Some of the foreland basins again, like the one in north-eastern India, can be looked


241

upon as a part of the adjacent subduction complex. Shannon and Naylor (1989) have cited the history of development of some of the complex basins in China as examples of composite basins. We discuss below the history of development of the Bengal Basin, located in eastern India and adjacent Bangladesh, as an example of a basin with a multiple history. The Bengal Basin occupies an area of more than 90,000 km 2, covering most ot the present West Bengal, India and Bangladesh. The basin is open to the sea in the south. Extensive geophysical surveys and deep drilling in the alluvium-covered plains of Bengal have revealed a thick sectiqn of Cretaceous and Tertiary sediments on a basement of basaltic lava flows. The lava flows are of the same age as the Rajmahal Volcanics (- 100 m.y.) exposed on the north-western and north-eastern fringes of the basin. Geophysical surveys also suggested the presence of Gondwana sediments in several block-faulted troughs below the basalt flows. A series of fault-bounded buried basement ridges mark the western fringe of the basin. East of this marginal zone of fault lies the shelf area of the basin with a low, homoclinal dip towards the south-east. The total thickness of Mesozoic and Tertiary sediments on this shelf exceeds 9 km in the eastern part of West Bengal and is of the order of 16 km in the deepest part of the basin located in south-eastern Bangladesh (Fig. 8.21). Except for a few normal fa~lts this sequence is structurally undisturbed. The most prominent marker horizon within the whole sedimentary sequence of this basin is a bed of Nummulitic (Eocene) limestone. This bed dips uniformly at the rate of 1Vzo over the whole of the Bengal shelf but in the subsurface of Kolkata exhibits a conspicuous basinward flexure traversing the entire basin in a NE-SW direction. Beyond this zone of flexure, which has been interpreted as the 'hinge zone' of the basin or as the Eocene shelf break, lies the deeper part of the basin with a greater rate of subsidence and a more argillaceous lithofacies. In the younger Tertiary sediments a similar change in lithofacies, from the arenaceous sediments of the stable shelf, to a dominantly argillaceous, deltaic sediment downdip, is noticed (Fig. 8.22). Except for phases of marine transgression occurring during the Upper Cretaceous (locally), Eocene (extensively) and Miocene (in the eastern parts only), the Tertiary sedimentation in this basin took place under freshwater, estuarine or deltaic conditions. The broad division of the Bengal basin into three major zones, (I) a western scarp-fault zone, (it) a stable, middle shelf zone, and (iii) a deeper basinal area (Sengupta 1966a) is conftrmed by more recent deep seismic sounding (DSS). This study also shows the presence of nearly normal thickness of continental crust in the western part of the basin. This changes to a significantly thinned continental crust in the eastern part (Kaila et al. 1992, Reddy et al.1995). The evolution of the Bengal Basin took place in several stages. Block faulting within the Bengal plains and the adjacent areas was followed by deposition of freshwater Gondwana sediments of Permian to Early Jurassic age. During the Early Cretaceous a major rifting of the Bengal plains took place consequent to the break-up of Gondwanaland and the drifting away of Antarctica from India. The' failed arm' of the aulacogen presumably rifted the entire Bengal plains while the other two arms mark the present boundaries of the eastern Indian coastline. The deep-seated faults located by seismic surveys between the Rajmahal and the Garo Hills (Rao and Sengupta 1964) are perhaps the only detectable remnants of this 'failed rift' in the subsurface.

Introductiol'l to Sedimentology

242

D

HIMALAYAS

SedIment, below cover at Rcu.nt alluvIUm ThIck (> 4 km) HIghly deformed lertiary (exposed)

fT:'Tl

Igneous and

~ metamorphlr basement AXI~ of the TertIary basIn

20°

•

t

• Contours IndIcate approx thIckness of TertIary sedIments In kilometres

RegIOnal cross-section

INDIAN

~ OC~ I: ::1 DeltaIc/Manne (OLIGO-PLEISTOCENE) ~ Manne (turbIdItes) PALAEO OLIGOCENE

oI

500km I

D

Shelf

E:::J Pelaglcs

I

CRETACEOUS-OLIGOCENE

Fig. 8.21: Tertiary baSins of Bengal and adjacent areas (compiled and redrawn after Paul and Llan 1975)

I

10 I

20 Miles

Semi continuous reflection

Horizontal Scale Vertical Exaggeration 18X

I

o

t:

"

.::

V

"o

Continuous (marker) reflection

SHELF

STABLE

t===-====== t ::r: g

DEEPER

~

~

~

""",-;.

"~\

~\

t;,

\

"---..

~

'-

~.-' ;,o".~ASIN

<'~,

;y

'"

t t ----t00::~

""

t

B

-18000

--15000

-12000

Q

fl..l

p..

::r: r

-9000 Z

~

fl..l fl..l

-6000 r

-3000

Level

0 Sea-

Fig. 8.2;l: Gross-section across western part of the Bengal Basin showing continuity of the seismic reflection horizons in the subsurface (redrawn from Sengupta, 1966a). With permission of American Association of Petroleum Geologists.

6000

3000

oj

A

~

o

0

w

~

I\)

:::I

~

:::I

(1)

3'

c.

(1)

en

:J C.

Dl

1/1

0'

0 :J

Ci 0

244


Rifting was associated with extensive eruption of flood basalt throughout the Bengal plains with the Kerguelen Hotspot possibly serving as the heat source (Mahoney et aI, 1983; Baksi et at, 1987). Tbe eruption of basalt was followed by freshwater, brackish or estuarine sedimentation over the Bengal plains, This situation continued until the Lower Eocene when extensive marine transgression caused deposition of Nummulitic limestone over the whole Bengal plains extending north-east up to Assam, In the regressive phase which followed, most of the basin experienced deltaic sedimentation from the Miocene through th~ Pleistocene, when an enormous thickness of essentially argillaceous sediments was deposited, particularly in the south-eastern parts of the basin (Sengupta 1966a; Roybarman 1983). This phase of deltaic sedimentation is apparently continuing in the south and south-eastern parts of the present West Bengal and Bangladesh. Offshore explorations suggest continuation of the Bengal basin sediments southwards, into the present Bay of Bengal. The upper three sediment layers of the deep-sea fan in the Bay of Bengal are presumably the offshore continuation of the Miocene, Pilocene and Quaternary sediment layers of the Bengal basin inland (Curray and Moore 1971).

9 STRATIGRAPHY AND SEDIMENTATION

CLASSICAL CONCEPTS IN STRATIGRAPHY Estimation of Geological Time The ultimate aim of a sedimentological analysis is to unravel the history of depositional events leading to the development of particular stratigraphic sequences. Studies on sedimentology and stratigraphy are, therefore, inseparable. Evolution of the planet earth, both in the organic and inorganic sphere, through geologic time, has been the main concern of stratigraphy. Estimation of geological time, both in relative and absolute terms, has naturally been a matter of prime' concern for stratigraphers as well as sedimentologists. Steno's Law of Superposition, and Cuvier's studies on faunal assemblage made estimation of the relative ages of rock layers possible even in the early years of the nineteenth century. A layer of sediment occupying a higher position than the others, as also a layer containing an assemblage of organic remains with advanced anatomical features, is clearly younger than the rest. A novel method of age determination in absolute terms, introduced by Charles Lyell, was based on the principle that the younger rock layers should contain larger proportions of living organic species compared to extinct ones (see Chapter 1). In an early method of age determination in terms of years, Lord Kelvin, a physicist, assumed that the earth had cooled at a uniform rate from a state of white heat to a condition wherein condensation of water vapour in the form of oceanic water was possible. The age of the earth obtained by this method in the late nineteenth century was a gross underestimate because the process of radioactivity, which replenishes much of the heat lost through radiation, was still unknown. This phenomenon, discovered jointly by H. Becquerel and Marie and Pierre Curie in 1896, and explained by Rutherford and Soddy in 1902 in terms of transmutation of atoms with a constant 'half-life', provided, for the first time, a dependable method for age determination of rocks in absolute terms. The radiometric method of dating is very much refined today and a host of radioisotopes, such as U238 ---; Pb 207 , K40 ---; Ar4 o, Rb 87 ---t Sri>?, are employed in radiometry. A similar method, using the ratio of radiogenic (CI4) and non-radiogenic carbon (C12 or CI3), can date woody matters, which are less than 50,000 years old. A more recent method of dating sediments uses the principle of magnetostratigraphy. Magnetically susceptible particles in sediments occurring in the deep ocean-floor 245


246

reorient themselves, in a statistical sense, to the directIOn of the earth's magnetic field eXisting at the time of their deposition. By matching the magnetic polanty of an ancient sediment With the standard, radiometncally dated, magnetic stratigraphic column, some Idea of the time of deposlton of the sediment can be obtained Problems are often created by bIOturbation causing reorientatIOn of sediment grams by deep-water burrowing orgamsms. On land the technique of magnetIc stratigraphy can be used for datmg ferrugmous detntal (non-authigemc) particles laid down under OXidizing conditIOns provided they have not been disturbed subsequent to deposition.

Dual Hierarchy in Stratigraphy By the end of the mneteenth century It was recogmzed that the strata conStituting the earth's crust could be grouped either as mappable rock units or on the basIS of the time span they cover. ThiS concept of 'dual hIerarchy' m stratigraphy has continued to influence all our stratigraphic thinking since then The problem of correlation between Visible rock umts and inferred time units was a difficult one. In the absence of a direct method for dating, the pioneers were obliged to use fossils as mdlcators of relative ages of rocks. ThiS practice was later challenged on the ground that growth in the organic world is controlled not only by the trend of long-ranging organic evolution, but also by short-rangmg local environmental conditions. Moreover, a parallel relationship between the rock and biostratigraphic units, an essentIal prerequIsite for dating of rock layers by their fossil content, may not eXist. In an elegant solution to this problem, spans of bme are recognized not by their fossil content, but by the deposits accumulated dunng specific time mtervals (e.g. 'systems' deposited dunng 'periods' of time). The Schenck and Muller (1941) scheme of clasSificatIOn of stratigraphiC units, reproduced in Table 9 1, IS based on thiS principle Although the geological time units and time-stratigraphic units are directly correlatable in thiS scheme, the rock umt terms have been delIberately written across to make direct correlation between rock and time-stratigraphic units Impossible. ThiS trend of thinkmg eventually led to the claSSification of stragraphlc umts into two distmct groups 1) observable units consistIng of lttho-and biostratigraphic units and 2) mferenttal units conslstmg of chronostratigraphic umts and geologiC time Untts This subdivision was the basis for creation of the Code 0] StratIgraphIc Nomenclature. The categories and ranks of stratigraphic units defined in a modern version of thiS code, as proposed by the North American Commission for StratIgraphic Nomenclature In 1983, is reproduced in Table 9.2 Table 9.1: Schenck-Muller (1941) Scheme of Stratigraphic Classification

GeologIC Time Umts

Era Penod Epoch Age

Time Stratigraphic Umts

Rock Straltgraphlc Umls ::=

System

0

Senes Stage Zone

0..

;:3

....0

0

c0:3 E .... 0

~

.... II.>

.c

E II.>

~

""co Il.)

SuperSUlts SUlte

Lrthodeme

Supergroup Group

FOlmatJon

><

Polanty Subzone

Polamy zone

Polanty Superzone

SubblOzone

(Interval, Assemblage or Abundance)

BIOzone

BlOstratlgraphlc

Polanty Superchrol)ozone

Eon

Era (Superpenod)

Eonothem

Erathem (Supersystem)

Epoch

Age (Subage)

Chron

Scnes

Stage (Substage)

Chronozone

* Fundamental umts are ItalICized

PerIOd

(Superpenod)

System

(Subsystem) Polarity Subchronozone

Polarzty Chronozone

Polanty ChronostratIgraphIc

Chronostratlgraphlc

Geosol

Pedostratlgraphlc

Po'anty Subchron

Polartty Chron

Polanty Superchron

Polanty Chronologlc

B TEMPORAL AND RELATED CHRONOSTRATIGRAPHIC UNITS

£:: 0 U

p..

~

Magnetopolanty

Geochronologlc Geochronometrlc

Member (or Lens or Tongue) Bed(s) or flow(s)

Llthodemlc

Lltho<;tratIgraphlc

A MATERIAL UNITS

0

6'"

u

.z;

\-<

c

Chne

Phase Span

EpISode

Dlachromc

Allomember

AllojormaJlOn

Allogroup

Allostratigraphlc

Table 9.2: Categories and ranks of units defined in the North American Stratigraphic Code (1983)*

.....

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:J

0"

S"

:J

-

3" m

a.

CD

en

a.

III :J

'<

::r

"'0

~

10'

(f)

-... a


248

The 'material umts' and the 'temporal and chronostratigraphic units' of Table 9.2 correspond respectively to the observable and inferential units of the conventional scheme. Of the material units again, the lithostratigraphic units are used for categorizing and ranking of deposits of sediments in the field. The fundamental lithostratigraphic unit, called a formation, is defined as 'a body of rock identified by lithic characteristics and stratigraphic position; it is prevailingly but not necessarily tabular and is mappable at the earth's surface or traceable in the subsurface.' Subsurface explorations in large sedimentary basins conducted during the last few decades by deep drilling and geophysical techniques have provided a new perspective to stratigraphic concepts. The conventional as well as the newly developing ideas are briefly summarized below. Readers may refer to Dunbar and Rodgers (1957) for a masterly treatment of the traditional stratigraphic concepts. The newer concepts are summarized by Walker (1984a) and Reading (1986) RELATIONSHIPS BETWEEN SEDIMENTARY UNITS' Vertical and Lateral Relationship

Vertical Relationship Sediments accumulate within basins by aggradation, progradation and lateral accretion. Aggradation is the process of vertical ftlling, as in the flood basin of a river. Progradation . is the ftlling from the margin, as in the case of an advancing delta front. Lateral accretion is caused by the process of accumulation against the margin of a channel, as in a river poin t bar (Figs. 9.1, 9.16). The vertical relationship between succeeding sedimentary units produced this way may be either conformable or non-conformable. A conformable relationship may be either sharp or gradational. Two types of gradations are possible: a continuous gradation or an intercalated contact. A body of sand grading vertically up into a shale section through mixtures of sand and shale in various proportions; is an example of a continuous gradation. A sand body passing vertically up into a body of shale through an intermediate layer of closely spaced lenticles of sand and shale is an example of intercalation. An unconformable relationship between two sedimentary bodies implies a break in the process of sedimentation on a large scale. This is generally caused by tectonic uplift of the area of sedimentatioIlleading t9 subaerial exposure and erosion of the previously deposited units. When the area subsides again, the newly deposited layers are said to be unconformably overlying the preceding ones. Various types of unconformable contacts are possible. When stratified rock layers overlie igneous or metamorphic bodies, the relationship between the two is said to be non-conformable. The unconformity in this case is readily recognizable. While mapping a non-conformity one should be certain that there is no intrusive relationship between the two sets of rocks. The possibility of contact metamorphism is also to be eliminated. When the older beds within a sequence of sediments are tilted or deformed, before being buried by the younger layers, the resultant break is called an angular unconformity. Such unconformities are easily detectable during field mapping. When the older and younger layers lie parallel to each other with only an erosional break between the two, the relationship is designated a disconformity. This is often marked by a layer of conglomerate conSIsting of eroSIonal products of the stratigraphically older unit. The

in

249

Stratigraphy· and Sedimentation

term paraconformity was introduced by Dunbar and Rodgers (1957) to describe the situation wherein a time .gap occurs without a detectable physIcal break between the upper and the lower units. Such time gaps between the two depOSItional units can be identIfied by palaeontological or radiometnc method (Fig. 9.2). Stream channel

AggradatIon

i

Finmg upward

® o

o II) 1I"'l..-

Marsh on surface ......

~r..-;;~--=:::-=''---'~-.,......;==-===...... =-.......... .,..-- - '-,

'--" .... , \

V

Sea-Level

'-

"

\

1 V

Coarsemng upward Fig. 9.1: Various mechanisms of sediment accumulation (A) fining-upward sequences produced In a flUVial regime by lateral accretion Within pOint bars and aggradation Within flood baSinS, (8) coarsening-upward sequences produced by progradation of a delta (based on IllustratIOns by Galloway and Hobday 1983 ,Scruton 1960 in Pettijohn 1975). Note 'blue pattern' In dlpmeter log at the left.

Recogmtion of the different types of unconformities can help m deciphenng the depositional history of a sedImentary basm. A non-conformable relationship IS expected at the basm fringe where layers of sediments overlte the igneous or metamorphIc basement. Penodic WIthdrawal of the sea from the deeper parts of a baSIn or a tern porary


250

phase of subaerial exposure, may produce a disconformable relationship between the underlying and the overlying layers. Paraconformable relationships are to be expected at the centre of the basin where the sedimentary pile is very thick.

- -------

--

--7--....:. . . . . . . . . . .

•

,..

l'

)II

•

Jl

----;-.....::::::ii:===:::j

T~

1\

•

JI.

,,10(

Non - conformity

--

--7

----'- -I

- .... I

.' ':-:-" . ......: -~ . .... ..... .. '. , . -.. '

Disconformity

'

,"

-

I J

I I

\

I

I

1

\

'

I

I I

I

I

t

J

I I

I

I

J

i ~

1 1 1

Paraconformlty

Fig.9.2: Four types of unconformities (based on Dunbar and Rodgers 1957).

In the case of a gradual marine transgression over an unconformity, the younger layers overlap the older ones and are onlapped by the still younger sediments. The younger layers in this case are progressively displaced away from the centre of the basin. In a typical case of marine transgression on land, this results in a fining-upward sequence. In the event of a gradual marine regression, the sea is progressively displaced .towards the basin centre and the younger layers are shifted away from the land area. The relationship between the successive sedimentary layers in this case is omapping. Pinchouts are common in onlaps but truncations occur frequently in oftlaps (Fig. 9.3).

Transgressive phase and overlap (onlap)

Regressive phase and oftlap

Fig. 9.3: Onlap and offlap relationships produced by marine transgression and regression. During a transgressive phase each unit onlaps the preceding one (based on Krumbein and Sloss 1963).

Stratigraphy and Sedimentation

251

WhIle an unconformIty represents a major break In sedImentatIOn, on a regIOnal scale over a protracted penod of time, many small breaks of short duratIOn (dlastems) may occur wlthm a basm EroSIOn and dIspersal of loose sedIments at sea bottom by sheets of movmg water, a process termed 'sublevatlOn' (Latm sublevare to hft up) by Dunbar and Rodgers (1957) IS an example of such a break

Lateral Relationship PhysIcal changes between consecutive sedlmeutary UllltS do not take place In vertical drrectIOn only Lateral changes III deposItIonal condItIons (,faCIes changes') may cause dIfferent but contemporaneous rock types and faunas to be deposIted m stnke contIlluatIOn of each other One of the claSSIcal areas III whIch demonstratIOn of such a change made the concept of faCIes VarIatIOn acceptable m North AmenqlIS the Devoman Catslall delta of New York State Careful mapplng m thts area m 1933 mdtcated that the Catsklll delta faCles and the Chemung Group of rocks are of the same age, although the Catsklll, restmg on top of the Chemung, appears to be younger (FIg 9 4) SImilarly, In the Grand Canyon of Colorado, Isochronous deposltlon of several Cambnan formatIOns was demonstrated by McKee (1945, 1949) Several such claSSIcal examples of faCIes vanatIon are to be found m Dunbar and Rodgers (1957) In the PermIan Barakar FormatIOn of the Southern Karanpura coalfield of India, Banerjee (1960) demonstrated how a rhythmIC pattern dommated alternately by coarse-gramed, sandy channel faCIes and fine-gramed (coal-shale) mterchannel faCIes could develop by shlftmg of anCIent nver channels GradatIOnal and mterfingenng contacts between homotaxlal channel and mterchannel Permo-TrIaSSIC flUVIal depOSIts were also estabhshed m the GodavarI valley of southern IndIa by Sengupta (1970, 2003, see FIg 7 27) Llthosome The rock umts mvolved m the process of lateral (faCies) change, lIke the SItuatIOns descnbed above, show up as three dimenslOnal masses of essentIally umform Itthology havmg a gradational or Intertongumg relatlOnshlp WIth adjacent masses of dIfferent lIthology Such a body of rock IS deSIgnated as hthosome Shapes of Itthosomes, defined by external morphology and three dImenSIOnal geometry (length, breadth and WIdth), are related to speCIfic tectolllc and envIronmental settIngs and may prOVIde clues to the mterpretatlOn of anCIent envltonmental condItIOns For thIS purpose faCIes aSSOCIatlons, or groups of faCIes, WhICh occur together and are genetIcally connected, are more Important than the geometry or character of an mdlvldual lIthosome FIg 9 5 tllustrates how the conventional hthostratIgraphic termmology IS to be appbed dUrIng mappmg of laterally or vertically occurnng faCIes aSSOCIatIOns Walther's Law FaCIes aSSOCIatIOns, WhICh occur SIde by SIde today, and pass laterally mto each other, WIll be preserved vertically above one another III geologIcal record Conversely, the lateral assoctatlOns of faCIes III geologIC past can be mferred from what IS seen In vertIcal stratigraphIc sectIOns today, usmg the LdW of CorrelatIon of FaCies formulated by Walther III 1884 ThIS law states" the vanous deposits of the same faCIes area and sImtlarly, the sum of the rocks of dIfferent faCies areas were formed beSIde each other

'--'--

Shale

----

'--:-.

. ..

-----=-~-

---=-

®

Limestone

-

~

100

80

Shale

.'::~:

60

Miles 40

20

o

&1-1000

.....
E

F=ig_ 9_4: Sections across the Devonian Catskill Delta, New York, showing stratigraphic interpretation: (A) before acceptance of the facies concept and (9) after introduction of the facies concept in the area by Chadwick and Cooper in 1933, The curved lines in 9 indicate time surfaces (simplifieil and redrawn after Dunbar and Rodgers 1957),

w

-'--' -

'

@

Catskill Group

'<

(C

o

m :::I

-a

3-

Q.

(!)

en

0-

0::s

o

C. c:

o

:;

-. -

N

(TI

N

253


-- - ..

r- : - -

C Shale B Sandstone'

•

r-

A Llmestone

®

P

..jr".' 01

~I

I

J

......

',I

....

-

,- G Shale :- - -

- -: G Shale J

1.l t

:"", I

':~·t"Jl-i:= ,

j

,

•

,

I'T' ~

F FormatlOn

~. E LlInestone

"

L

©I.~@,---

.

Sand~tone

__-·

K Shdle

:-

. -

--

- =---. -- .--- --. -...... :-.-. -----=- :- -.,-

®

EXPLANATION ~ Conglomerate

D

Sandstone

~ Stltstone F~~:::::=J Mudstone, Shale

~ Limestone ~ Dolostone (dolomite)

Fig. 9.5: Different types of lithostratigraphic boundaries In areas of facies change (redrawn from the Code of Stratigraphic Nomenclature proposed by the North American Commission 1983) (A) Boundaries at sharp lithological contacts and In laterally gradational sequence, (8) alternative boundaries In a vertically gradatIOnal or Interlayered sequence, (C) possible boundaries for a laterally Intertonguing sequence, (0) possible clasSification of parts of an intertonguing sequence, (E) key beds (R & S) are USI':O as boum\anes to O\s\\l'JgU\slJ Q,SIJa\~ Memb~r \rom \he o\hel parts of N-Formatlon A lateral change In composition between the key beds reqUires that another name, P,Sandstone Member, be applied The key beds are part of each member With permiSSion of American ASSOCiatIOn of petroleum Geologists.

m space, but m a crustal profile we see them lymg on top of each other, . Only those facies and facies areas can be supenmposed, wlthout a break, that can be observed beside each other at the present tlme"(translated by Blatt et at 1972, p 187- 188 from the origmalm German)


254

The slgmficance of Walther's Law can be explamed by refernng to the fluvial sedimentation model At any pomt of t1me fluVial channel sands coexist, and laterally pass mto, the finer gramed floodbasm (marsh swamp) depos1ts which m turn pass laterally Into deltaiC depOSits bordenng the sea (F1g 96) W1th progress of time and subsIdence of the basm, the sand bod1es cut mto, and are preserved partly on top of silt and clay belongmg to the floodplam faCies These m turn partially overlIe the deltaiC depOSits Complete preservat10n of such a sequence In geolog1c record IS possible only If the baSin of depOSitIOn subsides contmuously In fact, contmuous subsidence (wIthout any break) IS an essential prerequIsIte for the Walther's Law to be valId An example of detntal sand lenses preserved m vert1cal sequence due to contmuous slbsldence (with some oscillatIOns) of a flUVIal delta complex 1S 11lustrated III Fig 9 7

Sea

Land (1)

v

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(2)

..c: .....

..... ~ v u t:: v "d ..... (/)

.g (f)

Key to faCies ~ FlUVial

~ Deposits

D

Marshy Swamp DepOSits

DeltaIc DepOSIts

D

Manne DepOSIts

Fig 96: Diagrams Illustrating Walther's Law FaCies associations occurring side by Side today are preserved vertically above one another In geological record due to continuous subsidence of the baSin

P:7'72:d Marsh & Swamp

r:-·~ Detntallens

ilv//,?

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sedunents

Flood plam

Floodplam

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DeltaIc

7

SubsIdence

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Manne

PLAN VIEW

B

Manne SedIments

Present Shorelme

r/

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ASSOCiatIOn of Petroleum Geologists

cycles produced m a hypothetical delta complex by subsidence (after Coleman and Gagliano 1964, as modified by Weimer 1976 (Inset shows projection of the detrital lenses In plan view) With permission of American

Fig. 9.7: Overlappmg

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256


Cyclicity

In the example cIted above cycles of fmlng-upward sequences are repeated vertIcally by the process of channel mIgratIOn ThIS may take place without a major break during the process of sedimentatIOn Such repetItIOns are called autocycltc. RepetitlOns may also happen on a much larger areal and temporal scale due to, for example, tectOnIC or base level changes leadIng to superpositlOn ,of dlsconformlty bounded depOSIts. These are termed allocycles The North Amencan Code (1983) provides for claSSIficatIOn of these mappable allostratIgraphIc umts Into a hierarchIcal system of decreasmg rank allogroup, alloformatlOn, allomember (Table 92) The boundanes between these allocycles are produced by large-scale stratIgraphIC events related to regional sea-level fluctuatIons. Allocycles have been dIscovered m many coal basms of the world The Pennsylvantan cyclothem of IlhnOls and the Carbomferous cyclothem of YorkshIre, whIch are cited as claSSIcal e:Aamples, have been attnbuted to sea-level fluctuatIons, either due to penodlc dIastrophIsm or to waxmg and wamng of land glaCIers. Some authors have also attempted to correlate these penodlc fluctuatIons to the orbItal cycles of the earth's preceSSIOn (31 ,000 year cycles), obhqUlty (43,000 year cycles) and eccentriCIty (100,000 year cycles), dIscovered by Mllancovitch In 1941 and subsequently named after hIm Dunng the last two decades earth cycles of various magnitudes have been dIscovered. Thirtytwo-millIOn year cycles mvolving changes In the dIstribution of marine unconformItIes, patterns of faunal diversIty, occurrence of black- and red-clays have been recorded These have been attnbuted to cyclic changes in global climatIC condItions caused by vanatlOn m the rates of volcanic actiVIty at oceanic ridges (Fischer and Arthur 1977) These 32 m.y cycles are superimposed on much larger cycles of about 200 m y. duration SeIsmIC surveys have led to the discovery of major cycltc eroSIonal breaks of dIfferent magmtude during the Phanerozoic (FIg. 9 8) Of these, the major (first-order) breaks have been correlated to global sea-level fluctuatlOns caused by changes m ocean-floor spreading rates (Vall, MItchum and Thompson III 1977) These have some correspondence to the 200 m.y cycles of Fischer and Arthur The rise m sea-level could have been caused by buoying up of sea-water due to heatmg near the spreadmg mldoceamc ndges (Pitman 1978) The lower order sea-level changes have been attributed to glacio-eustatIc changes (Vatl et al. 1977) or, alternatively, to flexures during basin formation on an elastic crust (Watts et al 1982). Freezmg and lockmg up of sea-water in the form of land Ice would cause a generallowenng of the sea-level, but whether thIS would cause a transgreSSIon or a regresslOn of the shorelme would depend on the rates of sedIment input, tectOnIsm and IsostatIC adjustments A regreSSIon may take place even when the sea-level is high, If the sedIment mput is also hIgh at thIS stage Simtiarly, a transgreSSIOn may occur even when the sea-level IS low, If the rate of subSIdence of the. land area is fast, eIther due to tectontc or due to IsostatIc causes. The local sea-level changes therefore, are the net product of many factors of whIch eustacy, Isostasy, tectonIsm and sedIment lUput are more Important (see Readmg 1986 for a more detatled dISCUSS Ion)

257


2nd order cycles (super cycles)

Ist-order cycles RelatIve changes of sea-level Rtsmg Falhng

10

05

PERIODS

0

RelatIve changes of sea-level RlSlng Fallmg 1a a5

TERTIARY

100 I

100

eRE rACEOUS

JURASSIC -------

200

TRIASSIC

Present

PERMIAN

300

-------

00

PENNSYL VANIAN MISSISS IPPIAN

DEVONIAN

400 2

200

-- .. - ..

- .. - ..

-

00

SILURlAN ORD OVIC I ------

500

--------

00

PRECAMBRIA Fig. 9.8: Flrst- and second- order global cycles of sea level fluctuations during the PhanerozOIC Eon (after Vall, Mitchum and Thompson III 1977) With permission of American ASSOCiation of Petroleum Geologists

Two orders of transgressive regressIve cycles (called T-R cycles), the larger one correspondmg to the thud-order cycle of Vall et ai (1977) and the smaller (named fourthorder cycle) comCldmg wIth the transgressive-regressive maxima of Vat1's third-order cycle, have been Identified m the coal-bearmg Upper Cretaceous Strata of Utah (Reyer 1983) Discovery of short (metre scale) basmward dlscontmmtles wlthm the Helderberg Group or rocks (Devoman) m New York State, led some modern stratlgraphers to suggest that the stratigraphic record, bamng mmor exceptIOns, COnsists of thm, shallowmg-up cycles bounded by geologIcally mstantaneous me of base level The baSIC assumptIOn behmd thIs model IS that the mode of stratigraphic accumulatIOn IS epIsodiC rather than gradual, as tradItIOnally belIeved Goodwm and Anderson (1985), the advocates of thIS model, have demonstrated how epIsodiC depOSIts mIght be produced by eustatIC responses to orbItal perturbations supenmposed on contmuous basm subSIdence (FIg 9 9) They have also worked out In detail how (geologically) mstantaneous sea-level me followed

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THE PAC HYPOTHESIS

Fig. 9.9: (Y) Sequence of events leading to 'Punctuated Aggradational Cycles' (PAC). Each PAC IS bounded by a surface of abrupt change to deeper water facies. (X) Illustrates how this change may take place by sea-level fluctuation (after Goodwin and Anderson 1985). With permission of The University of Chicago Press.

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259

by aggradatlOn mIght lead to the development of what they termed Punctuated AggradatIOnal Cycles (PAC) Such a model of sedImentatIon, based on mferred, short hved epIsodes, precludes the apphcatIOn of Walther's Law except wlthm cycles The advocates of PAC belIeve that the PAC boundanes, bemg short-rangmg stratIgraphIc umts, wIll provIde the tool for detailed mtrabasmal correlatIon AnalysIs of CycliCIty

Rock umts, arranged apparently randomly In vertIcal stacks, may contaIn a hIdden regulanty ThIS regulanty, If deCiphered, may provIde clues to the mherent genetIc relatIonshIp between dIfferent facIes In a SImple method of studymg thiS relatIonshIp, de Raff, Readmg and Walker (1965) worked out the frequency of faCIes WhICh are m contact wIth each other m a vel trcal succesSIOn ThIS led to a 'spIder dIagram' deplctmg the facIes relatIOnshIp Nowadays, more sophistIcated methods are used for findIng the 'hidden motIf' wIthm cycles (MIa111973, Cant and Walker 1976) The procedure for quantltatrve analYSIS of CYClICIty, based on Gmgench s (1969) techmque for analYSIS of CYClICity III flUVial sequences, determmes the probabIlIty WIth whIch a certaIn rock type WIll follow another m the vertIcal column If the probable sequence of transitlOn, worked out m thIS way, does not gIve a complete pIcture, the 'hidden elements' may be detected by a second stage of analYSIS when the 'dIfference matnces' are computed Fmally, d test for the MarkOVIan property IS carned out In order to know If each depOSItIOnal event of the system was mfluenced by the event ImmedIately precedIng It, Ie, whether the depOSItIonal processes have followed a contInUOUS sequence The method outlmed above has been shown to be statistIcally Illcorrect (Walker 1984a), but the alternatIve procedure of analYSIS, claImed to be dependable (Harper, Jr 1984), IS too SOphIstIcated to be dIscussed here Expenence, however, has shown that the system of analYSIS proposed by Gmgellch (1969) gIves fatrly dependable clues to genetIc InterpretatIon of faCies m many cases A mechamcal procedure of analYSIS of cyclICity may fall to take mto account Important geological features such as the nature of contact between faCIes The conclusIOns which prove to be statistIcally sIgmficant, may, III some cases be geolOgIcally unImportant Moreover, every method of mathematIcal analYSIS based on summanzatlOn of the ongIllal observatIOns, runs the nsk of losmg valuable geological mformatlOn III the process The answer to thIS problem IS to return to the raw data at every stage of analYSIS Only the geologIcally meanIngful solutIons should be accepted (Readmg 1986)

Correlation Trad ItlOn ally, correlatIon ImplIes demonstratIOn of tIme eqUIvalence of particular stratigraphIc umts from the type sectlOns to the areas of theIr geographIC extensIon The cntena used for correlatlOn should, therefore, be mdependent of lIthology, fossIl content or other matenal basts of strattgraphlc dIVISIOn Some cntena for determlflatlOn of tnne which IS mdependent of all these should be used Although some authontles feel that In stratigraphy thiS narrow defimtlOn of correlatIOn should be retamed" (Rodgers, 1959) More modern texts advocate use of different types of cnterIa for correiatlOn (NIchols, 1999) These are as follows


260

2 3 4 5

CorrelatlOn of lIthostratigraphic units based on lithologiC cntena and stratigraphic posItIOn of the concerned rock bodIes Correlation of bIOstratIgraphIc umts based on fosstl content of the umts CorrelatIOn of magnetostratIgraphIc umts based on magnetic propertIes of rock bodIes dIfferent from those of the underlymg and overlYIng rocks CorrelatIOn of chronostratigraphIc umts based on time cntena That IS, a body of rocks whose upper and lower boundanes are Isochronous surfaces. CorrelatIOn of allostratIgraphic umts based on the pOSitIOn of a umt wIth respect to unconformitIes or theIr relatIve conformIties (see 'sequence stratIgraphy')

Technlqnes of Correlation Heavy Mineral Assemblages In the early days of geological exploratIOn heavy mmeral assemblages were often used for stratigraphic correlatIon The techmque IS based on the assumption that heavy mInerals are Incorporated Into layers of sedIments by progressIve de :-oofing of a source rock When such a process takes place at different geographIc locations, but WithIn a short span of time, the layers of sedIments may be taken as tIme eqUIvalent The method worked weIllll many cases For example, the proportIOns of epIdote, hornblende, and staurohte could be successfully used for correlatmg the Tertiary stratIgraphIc umts of Assam, Indld ThiS techlllque reached such a refinement that local stratIgraphIc megulantles could be detected by studymg the heavy mllleral assemblages of only a few odd samples (Evans et al 1934) The method of correlatlOn WIth the help of heavy mInerals alone has Its drawback. When the heavy mmerals occur wlthm a stratigraphic column III reverse order of their stabIlIty (I e when only the more stable mmerals are found III the older honzons), one may suspect that the less stable ones have been removed by crrculatmg mterstratal flUids An attempt of stratigraphIc correlatIOn by heavy mmerals IS of httle value under such a CIrcumstance (Pettijohn 1975) Wlrelme Logging

The best pOSSIble way of obtamIng subsurface lllformatIOn IS by dnllmg. Well cuttmgs contInuously brought to the surface by clrCUlatlllg dnl1111g fIU1d ('mud') do not prOVIde preClse mformatlOn on the nature of stratlraphlc contacts In the subsurface To obtam thIS InformatIOn core samples must be collected contmuously dunng dnlhng As thIS IS an extremely expenSIve process, a contmuous record of the electncal, somc and radioactIve propertIes of the sectlOns dnlled IS generally mamtaIned by wlrelmeloggmg Loggmg IS conducted by mtroduclllg lllto the borehole a sonde contammg an energy source and a detector The source IS a combmed current and potentIal electrode for e1ectncalloggmg, an ultrasollic transducer for somc loggmg, a neutron emISSion source for neutron loggmg, or a gamma ray (C0 60 , Cs 137) source for denSIty loggmg Recordmg IS done contllluously as the sonde 15 moved up the borehole (Table 9 3) The conventional electncal logs recordspontaneouspotenttal(S P) and res15t1vlly The former IS a measure of the electncal current generated at the contact of rock layers by mteractlon of the dnllmg mud With the formatIon water The latter measures the voltage drop across potential electrodes for known currents passed through current electrodes (Fig 9 lOA) ContrastIng salmlty of the pore flUId and the dnlImg mud IS essentIal for the generatlOn of spontaneous potential As the mud comes mto contact With sand or a


261

shale layer filled wIth salt-water, the electrochemIcal potentials add up to produce S P currents across the sand-shale sequence The smooth S P curve that generates agam~t a shale layer, when 1t 1S thIck, 1S called the shale Ime or base line Agamst porous sand beds, the S P curve sh1fts to the left The spontaneous potentIal generated 1S related to res1stlVlty of the formatlOn water (R.v) and the reSlShv1ty of the mud filtrate (RlIl{) Hence the S P log can be used not only for delmeatmg the bed boundary and the bed th1ckness, but also for est1matmg the resIst1v1ty of the formatlOn water Under sUItable condIt10ns the S.P curve can also be used as an md1cator of the shale content of a sand layer The h1gher the shale concentratlOn, the lower the amphtude of S P (F1g 9 lOB) Table 9.3. Log types, properties measured, and geologic uses (abridged from Cant 1984)

Log

Property measured

Units

Geologic uses

Spontaneous PotentIal

Natural electIlc potentIal (compared to dnlhng mud)

MillIvolts

Lithology (In some cases), correlatlOn,curve shape analYSIS, IdentificatIOn of porous zones

ReslstlVlty

ReSIstance to electrIc current flow

Ohm metres

IdentificatIOn of coals, bentonttes, flUId evaluatIon

Gamma-ray

Natural radloactlVlty ofK, Th, U

API umts

Somc

VelOCIty of propagatIOn of sound wave

Microseconds I metre

CaJtper

Dtameter of hole

CentImetres

Lithology (shahness), correlatIOn, curve shape analYSIS I dentlflcatlOn of porous zones, coal, tightly cemented zones Evaluate hole conditIOns and rehabilIty of other log~

Concentrations of hydrogen

Percent porOSIty

Identification of porous zones

Bulk densIty (electron denSity) mcludes pore

Ktlograms per CUbIC metre

IdentificatIOn of hydnte, hahte, non-porous carbonates etc

flUId m measurement

(kg/m3)

ReSIstIVity changes IndIcatIng dIPS

Degrees (and dlfectlOn)

Neutron

(water and hydrocarbons) In pores DenSIty

Dipmeter

Structural and stratigraphiC analYSIS, environmental mterpretatlOn

Smce S.P IS generated by the dIfference m concentratIOn of electrolytes m the mud and the formatIOn water, there will be no S P. 1f the salt concentratlOn IS of the same order m the mud and the formatIOn m contact In such a case a gamma-ray log may be used for determmatIOn of the sand-shale boundary. Shales contammg K40 Isotope prOVIde a natural source of gamma-ray flux In fact, a combmat1on of a number of logs (S. P., somc, denSIty and neutron lags) 1S used for accurate determmatlOn of the proportIOn of shale m a sandy shale The combmatlOn of gamma ray and somc logs 1S perhaps the best for the purpose of subsurface correlat1on (F1g 9 11) The somc log 1S effectIve m detectmg very h1gh velOCIty rocks (e g , carbonates or well-cemented calcareous sandstones) and also very low veloc1ty rocks (e g , coal, porous or fractured rocks WIth mterstltwl flUld, unconsohdated sedIments) The gamma-ray log, by detectlllg the shalmess of a rock, may md1cate whether a sequence IS 'coarsemng' or 'fimngupward' although thIS log does not measure gram-sIze

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RESISTIVITY

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showing S.P. and Resistivity (Short, Long-Normal, and Lateral) patterns for different lithological units within Locations of side wall cores are shown by $.

a borehoie.

Fig. 9.10: (A). Typical Spontaneous Potential (S.P.) and Resistivity patterns for sand, shale and limestone. (B). An electrolog

Shale

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Dolomite

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263


Gamma-Ray (API UnIts)

oI

SOnIC Interval Travel Tune

150 I 300

100

70

40

CalIper (mches)

160

130

100 Dommant LIthology

150

o

(mlcrosecs/ft ) I

16

I

I

I

I

fl

6400

Comment~

Mawly Sandstone

lc=====-_

Coal Sand,tone ("llh conglomerate)

Four Sequences Capped by CoalCoal 6500 .p===iii~:: 2 known to be Sandstone RegreSSIve Manne(wIth conglomerate) to Nonmanne Cycles from Core --:=::::::"oIo...L6600 ~=::::::=::;;~==t=~C~o~a~1==j1~ Sandstone and Shale

670°1=========~~~===~==~c~o~a~I====~~ Shale

13900

LImestone and Shale

14000

LImestone wIth Shale Interbeds

(

Two Sequences of Upwardly Increasmg Cal bonate Content

No Trends

(

Lower VelocIty

Cleaner )

More Shaly

)

HIgher Velocity

Fig. 9.11: Gamma-ray and sonic logs against coal and some tYPically siliciclastic and carbonate sections Reprinted from Cant 1984 with permission of Geological ASSOCiatIOn of Canada

The eqUIpment for reslsttvlty measurement m boreholes consists of a current and a potential electrode An electnc current IS sent through the current electrodes and the potential difference between the two potential electrodes IS measured. The greater the spacing of the electrodes, the deeper the penetratIOn of the current mto the formation. By usmg electrodes of diffelent spacmgs (Normal 16", Long Normal, 64", Lateral, 18'8") resistiVIties of the mud- mvaded 70ne and also the true formation resistiVity beyond

264


the zone of flushing can be measured (Fig 9.IDB). ResIstIvity measurement can aId particularly m correlatmg the carbonate beds, WhICh have high reSIStIvity and low porosIty A well-cemented calcareous sandstone, WhICh IS more resIstive than a porous, non-calcareous one, may be detected by resistIVIty loggmg. Both somc and resistIVIty logs may be used for detectmg the fluid content of a rock ReSistIvIty measurements usmg dIrect current work only when the drilling mud IS conductmg In empty wells, as also m wells fIlled With oil-based drillmg flUIds or gas, reSIstiVIty IS measured by transmItting alternating current mto the formatIon. The amount of current mduced Into the system IS a measure of the formatIOn reSistIVIty ThIS IS the method of mductlon logging. ReSIstIvity logs may also be used for measurement of the dip of a formatlOn withm a borehole. For thIS purpose mlcroresistivlty IS measured simultaneously by four (five in the case of hIgh resolution measurements) IdentIcal rmcroreSlstlvity electrodes mounted on four equally spaced pads of the dipmeter tool mserted mto the borehole A dlppmg bed produces a resistivity record of comparable magmtude at different depths on different walls of the borehole. Mathematical correlation methods are then applIed for measurement of these dIsplacements. IndiVIdual features are matched together for computatlOn of the dIP and aZImuth of the beddmg. These computatIOns, as well as correctIOn for the effect of deVIation of the borehole, are done by a mIcrocomputer bUIlt mto the system The dtjJmeteroutput is m the form of an arrow plot The combmed dipmeter-mclinometer plot, which shows dIP as well as azimuth IS called a 'tadpole plot.' Vertical vanations ill dIPS recorded m a dlpmeter log help in long- and short-interval correlation. Structural and stratIgraphIc features can also be detected in the subsurface by dlpmeter loggmg The methods commonly used for correlatIOn of borehole logs are: tracing of marker beds, sequence analysis and slicmg (Cant 1984) A bed havmg a dlstmctive log character may be used as a marker bed for the purpose of correlatIOn when It has an appreCiable areal extent The depOSits m low-energy enVironments, such as lakes and oceans, whIch generally extend over large areas, make good markers Those m high-energy enVIronments, such as rivers and sea beaches, are unperslstent and lack dIstmctive characters Such beds may be correlated for short distances with reference to the markers that may occur Immediately above or below. In the Nahorkatlya Oilfield in Assam, for example, a number of multIstory sand bodies, each of small areal extent, were mapped between two well-estabhshed marker honzons by S P. and Short Normal logs (Fig 912) Sequence analYSIS m wlrelme logs mvolves matchmg of readily recognizable vertIcal patterns such as coarsenmg or finmg-upward sequences by theIr log characters Such sequences, called 'events', may match palaeontologically establtshed time lines but they often cut across ltthological or faCIes boundanes When all other techmques of correlation fail, a log interval may be subdiVided arbitrarily mto slIces for studYing the nature of variation withm small mtervals. Thick deltaiC deposits, for example, may be correlated over large areas with the help of the overlYing and underlymg markers but the mtermedlate thIck pIle of sediment is 'sliced' to study the nature of facies vanatlOn withm short vertIcal dIstances. InterpretatIOn of faCies or depOSItIonal envIronment from the shape of log signatures alone IS nsky because no log pattern IS umquely related to any partIcular environment FIg 9 13 gIves the IdealIzed gamma-ray or S P. patterns found m some of the common depOSItional enVIronments Each log pattern shown m thIS diagram can generate m

SN

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SP SN

"'.,:,:.'

Marker Honzo n

.......

-

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Marker Honzone

.. ~

SP

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Fig. 9.12: Correlation of fluvial channel sands In the subsurface of Nahorkatlya, InOla by Spontaneous Potential (SP) and Short Normal (SN) resistivity logs (based on Azad et al. 1971). With permission of Schlumberger Technical SerVices, Inc.

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266

any of the deposltlOnal condItions mentIOned In choosmg between them one must utIlize some other mdependent line of eVidence connecting It to a genetic (faCIes) model

o

150

A

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150

B

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150

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150

D

Fig. 9.13: Idealised gamma-ray and SP log patterns In deposits of different types. Note that similar curves may be produced In different environments (based on Cant 1984). (A) Cyllndncal pattern Indicates clean sand of eolian, braided flUVial, carbonate shelf, reef or submarine canyon-fill origin. (8) Funnel-shaped pattern indicates coarsening-upward sequence likely depositional enVIronment crevasse splay, flUVial dlstnbutary bar, barner Island, submarine fan lobe. (C) Bellshaped pattern Indicates fining-upward sequence In pOint bar, deep sea channel, transgressive shelf sand. (0) Symmetncal pattern indicates sandy offshore bar or transgressive shelf sand

For example, a 'cyhndncal' log pattern (Fig. 9 .13A) can develop from a fluvIal sand within a distnbutary channel m a delta sequence, a carbonate shelf or a reefal growth in a shallow manne environment Therefore, before Interpreting the log SIgnature It IS important to know whether one IS dealmg With a marine or non-marine sequence. Direct Information on lithology, granulometry and sedimentary structure, as they be obtamed from core samples, is mvaluable for this purpose In the absence of borehole cores, sIde-wall cores may be extracted from deSIred locations of the borehole by specIal techniques.

Seismic Correlation Selsmlc surveys are conducted by detonatmg a charge near the surface and by detecting the waves reflected back to the surface from subsurface mterfaces. By convertmg the two-way travel times of elastic waves mto depths with the help of mterval velocIties, accurate mformatIOn about subsurface diSposItion of the reflectmg hOrIzons can be obtained by thIS method The data obtained are utilized for subsurface structural mappmg, correlation and faCies analYSIS. Modern techmques of seIsmic recordmg and dIsplay allow visualizatIon of the subsurface picture over large areas


267

For the purpose of geological mterpretatlOn composite cross-sections runnmg across the basm under study are constructed uSlllg seIsmiC reflectIOns One such composIte profile prepared for the western part of the Bengal Basm by means of seven seIsmIC reflectors IS shown m fIg 822 Two of these reflectors, one emanatmg from the top of an extensIve basalt flow underlymg the TertIary sedIments CR' reflector) and the other correspondmg to the mterface of a hmestone and a shale bed CD' reflector), were used as markers for geologIcal mterpretatIOn The seismIC SIgnatures of both the reflectors could be correlated over iarge areas Five other reflectIOns from shallower horizons termmated at dIfferent distances baslllward ThIS phenomenon was mterpreted as a basmward change of hthofacles The type or regIOnal correlatIOn achieved m the Bengal Basm by co-ordmated electro-loggmg and seismic survey IS Illustrated m FIg 9 14 The conventIOnal techlllque of seIsmiC mterpretatlOn expects the reflectlOns to correspond to hthoiogical Illterfaces charactenzed by sharp contrasts m acoustIc Impedance (product of veloCIty and denSIty) Marker reflectIOns generated from such mterfaces of reglOnal extent are charactenzed by then coherence over a number of seismograms, The 'D'-and the 'R'-reflectors m the Bengal Basm are two examples of such reflectIOns Apart from the marker reflectIOns a number of mmor ones, apparently not connected with strong hthoiogical mterfaces, are also found m every sedimentary basm Such a SItuation IS depicted III Fig 9 15 SeismIC reflectIOns represented by thick black hnes m thIS dIagram follow stratal surfaces which are anCient depOSitIOnal planes, correspondmg to time hnes These reflectIOns cross rock formatIOn (llthofaCles) boundanes SeIsmiC techntques have then hmltatlOns Subsurface rock bodies havmg thickness less than the wavelength of seismic waves go undetected Thus the umts seen m seismIC profiles represent packages of beds Moreover, the wave propagation veloCltles for shales, sandstones and hmestones overlap at depth, thereby makmg the seismIC method unSUItable for detectIOn of theIr Illterfaces (Gardner, Gardner and Gregory 1974) UnconformitIes are often the best seismIC reflectors Angular unconformities mvolvmg sharp changes m dip are the easiest to detect by seismiC surveys Even when angular changes are not mvolved, an unconformIty may be detected by the contrast m acoustic Impedance created by the presence of weathered zone at the plane of erosion In fact, unconformlues correia table on a global scale by seismic reflectIOns, and interpreted to be the effect of global sea-level fluctuatIOns, have gIven rise to a new concept m stratigraphy called sequence stratIgraphy SEQUENCE STRATIGRAPHY History

In 1941, L L Sloss Identified unconformIty-bounded packages of sediments of cratonwise extensIOn III Montana, USA. He named these packages 'sequences' III hthostratlgraphlc sense (see Sloss 1988 for a reVIew of these developments) Recogmtlon of unconformities of regIOnal extent and IdentificatIOn of eustacy as the dnvmg mechamsm for generation of unconformity-bounded sequences, proVIded the ground

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Stratigraphy ~nd Sedimentation

269

for revIval of the concept of sequence stratigraphy (Vail 1987, Haq et al 1987) At the same tIme development of high-resolutIOn seIsmiC techmques at the EXXON Laboratones at Houston provided the tools for IdentificatIOn of unconformity-bounded sedimentary sequences III tpe subsurface Thus, a global chronostratigraphic correlatIOn scheme, mdependent of formation boundanes and rock types, could be establIshed from physical stratigraphic analYSIS This led to the foundatIOn of the sequence stratigraphiC technIque, as we know It today (Sloss 1988) CLIMATE

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Alluvial PI am Sediments

Fig. 9.15: Diagram Illustrating development of stratigraphy as a funClton of sediment supply, subsidence, sea-level change (eustacy) and climate Thick black lines indicate seismic reflections (based on Vall, 1987) With permission of American ASSOCiation of Petroleum Geologists

Basic Concepts To have a grasp of the sequence stratIgraphiC concept It IS Important to remember that preservatlOn of a sedimentary sequence m nature depends largely on two factors, the rate at which sediments are aVaIlable, and the rate at which new space IS created for preservatIOn of these sediments The sediments mclude the clastiCS as well as the non-clastiCS The space for accommodatIOn of these sediments IS created either by subsidence of the basm of depOSitIOn, or by a nse III sea-level due to eustatic changes Subsidence of land due to tectomc movements and transgresslOn of sea on land due to eustatic changes will have the same effect on sedImentatIOn The followmg three condItlOns may anse out of the combmatlOn of subsidence and sea-level changes 1 When the sediment supply equals the space aVailable for ItS accommodatIon, the depOSits buIld up vertically The phenomenon IS called aggradatIOn (Fig 9 16A) 2 When the space avaIlable for accommodatIon of the sedIments grows faster than the rate at which sediments are available, the sedimentary sequence shows retrogradation (Fig 9 16B) ThiS happens dunng landward movement of the shorelme


270

(transgressIOn) 3 When the volume of sedIments exceeds the space avallable for Its accommodatlOn, an upward change to a shallower facIes takes place wIthm the baslll. ThIs phenomenon, called progradatzon, takes place dunng regressIOn, when the shorelIne moves away from land (FIg. 9.16 C). Many of the stratIgraphIc features seen m reglOnal seismIc profiles are the results of fluctuatIOn of sea-level Of these the followmg are more Important (FIg. 9.17) Steeply lllc1med surfaces called clinoforms are produced when progradatlOnal sedIment packages buIld layers mto deeper water QjJlaps are stratal packages bUlldmg upwards and outwards into the basm TopJaps are created when a succeSSIOn of packages of sediments prograde basmward without aggradatIOn. DownJaps are lllclmed surfaces termlllatmg abruptly on horizontal surfaces lymg below. These structures are the results of sedImentatIon under marine condItion, but some structures of non-manne ongm may resemble these features For example, Gilberttype deltas, and also some fore reefs may show downlaps. Dlfferentlal compaction of mudstone and sandstone at the basin edge may appear as downlaps in reglOnal seIsmIC profiles because thm Ulllts of mudstbnes may go undetected.

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Fig. 9.17: Stratigraphic features recognizable In regional seismic profiles. (Based on Nichols 1999) W,th permission of Blackwell SCience Ltd., a Blackwell Publishing Company


271

Terminology The EXXON Group coined a new set of terms to designate the sedimentary sequences generated out of subsidence and sea-level fluctuations. Following is a list of some of these terms and their definitions (Mitchum et at. 1977 & 1977a). A depositional sequence is defined as a "relatively conformable, genetically related succession of strata bounded by unconformities or their correlative conformities". This sequence represents the sedimentary unit deposited between two episodes of significant sea-level fall. A fall in the sea-level exposes the beach and foreshore, subjecting them to subaerial erosion. The unconformity thus developed may cover whole of the shelf area if the fall in the sea-level is significant enough. This unconformity, recognized in seismic profiles by termination of a series of continuous reflectors against a surface at high angles, is reckoned as a sequence boundary. In the outer shelf, where there' is no erosion, the surface equivalent to this unconformity is the 'correlative confonnity' in Mitchum's definition. While the sequence boundaries are created by events of major sea-level fall, bodies of sediments may be deposited during phases of intermittent sea-level rise between two major events of fall. Several types of systems tracts may develop out of these movements. Systems tract, the fundamental mapping unit in sequence stratigraphy, is defined as a three dimensional assemblage of several lithofacies which are the products of same processes or environments. This means that they have to be genetically linked. Like the Code of Stratigraphic Nomenclature, the Sequence Stratigraphic scheme proposes its own hierarchy of stratal units. The fundamental unit, parasequence, is defmed as "a relatively conformable succession of genetically related succession of beds bounded by marine flooding surfaces and their correlative surfaces" (Van Vagoner et al. 1990). A para sequence set is "a succession of genetically related parasequences that form a distinctive stacking pattern, bounded, in many cases by major marine flooding surfaces and their correlative surfaces. Parasequence and parasequence set boundaries form in response to an increase in water depth. Under certain depositional conditions para sequence and parasequence set boundaries may coincide with sequence boundaries" (Van Wagonar et at. 1990). Parasequences are composed of bedsets, beds, laminasets, and laminae. "Each stratal unit, with the exception of the lamina, is a genetically related succession of strata bounded by chronostratigraphically significant surfaces, and each surface is a single, physical boundary that everywhere separates all of the strata above from all of the strata below over the extent of the surface ... Because of these properties, bounding surfaces that are correlated using well logs, cores, or outcrops, provide a high-resolution chronostratigraphic framework for facies analysis" (Van Wagoner et al. 1990). Time range, thickness and areal extent of the stratal units in sequence hierarchy are shown in Fig. 9.18. The characteristics of lamina, laminaset, bed, and bedset are given in Table 9.4. It may be noted that some of these terms duplicate similar terms in the Code of Stratigraphic Nomenclature but carry different meanings, thus leaving a scope for confusion. Depositional Sequences

The sequences of beds referred to above, occur in cycles because they are related to the cyclic patterns of sea·level fluctuation. A complex pattern of sinusoidal curve is generated by superimposition of the short-term sea·level fluctuations on longer-term

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DtifimtlOn

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Untform In composltlOn /texture Never mternally layered

ConSISts of a group or set of conformable lammae that compose dlstmCtIVe structures mabed

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PenodlC deposItIOn mcludes deposltlon from seasonal or chmatic changes EpisodiC, commonly found In wave- or current nppled beds, tufbIdIt<:;S, wave nppled mtervals m hummocky bedsets, or cross-beds as reverse flow rIpples or rIppled toes of foresets EPisodIC

EpIsodIc deposloon mcludes depOSItIon from storms, floods, debns flows, tubldlty currents

EpIsodic or penodlc

• Form very rapidly, mmutes to hours • Smaller areal extent than encompassmg bed

• Form rapidly, mmutes to years • Separate all younger strata from all older strata over the extent of the surfaces • FaCIes ehanges are bounded by beddmg surfaces • Useful for chronostratigraphy under certam ClTcumstances • TIme represented by beddmg surfaces probably greater than Hme represented by beds • Areal extents vary widely from square feet to 1000s square miles • Form rapidly, mmutes to days • Smaller areal extent than encompassmg bed

(Same as Bed below) plus Bedsets and bedset surfaces form over a longer penod of tlme than beds Commonly have a greater lateral extent than beddmg surfaces

CharactertStu::s of boundmg surfaces

Beds above and below bedEPIsodIC or penodic (same sets always differ III compo- as bed below) • slOon, texture, or sedimentary structure from those compo• smg the bedset

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Table 9.4: Detailed Characteristics of lamina, laminaset, bed and bedset (from Campbell 1967)

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274


cycles (Fig. 9.19). High- and Low-stand systems tracts occur at the crest and trough positions of these cycles respectively. Parasequences are deposited during shorter-term sea-level fluctuations within the longer term ones. The bounding planes of these parasequences are the marine flooding surfaces. A depositional sequence consisting of a highstand and a lowstand systems tract generally includes a number of parasequences.

Facies Associations Three of the major situaJions related to falling sea-level, rising sea-level, and high sea-level are discussed below.

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Time-'7 Fig. 9.19:Diagram illustrating development of high- and lowstand systems tracts (S.T.) and parasequences consequent to relative sea-level rise and fall (based on Nichols 1999). With permission of Blackwell Science Ltd., a Blackwell Publishing Co.

Case 1. Falling SeaLevel: Lowstand Systems Tract

Deposits laid down immediately ~fter a relative fall in sea-level but before the next phase of sea-level rise represent lDwstand systems tracts. A falling sea-level lowers the base level thus affecting the river systems at the shelf edge. With lowering of the base


275

level rivers incise into their floodplains and carry the products of erosion into the sea where they are redistributed by turbidity currents. As a result the submarine fan lobes grow fast and change vertically to shallower facies. Progradation of coarser turbidites mark the sequence boundary in the deep sea (Fig. 9.20A). The products of erosion of the hinterland develop a blanket over the canyons on the continental slope.

systems tract

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LITHOLOGY ~ ~

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Fig. 9.20: (A) Lowstand Systems Tract (LST) and (8) Highstand Systems Tract (HST) produced respectively by sea-level fall and rise. Note the timing of the model with respect to the sea-level sine curve (based on Posamentier et al. 1988).

276


In areas showing ramp geometry (a gentle slope without a sharp break m profi1~) the zone of erosion IS limited but the faCies associations show a distinct ShIft towards the basin. Under thiS condition, a sequence boundary IS defined as the plane above the falling stage systems tract (Fig. 9.21A). A stage of fallmg sea-level is usually followed by a phase of slow rise in the relative sea-level. Topsets and clinoforms are produced during this phase. In limestone bearing areas, a fall in sea-level leads to karst formation on land when the climate is humid. In arid climate, it leads to hypersaline conditions developing sabkhas in coastal areas.

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Fig. 9.21: Development of systems tracts at shelf edge follOWing sea-level rise and fall (based on Illustrations by Van Wagoner et al. 1990).

Case 2. Rising SeaLevel: Transgressive Sysems Tract A transgressive systems tract IS produced when the sea-level rises and the coastal plains are inundated. The sequence boundaries, under such a condition, may be recognized


277

by IllClSIOn of fluvIal deposits IlltO the shelves Estuanne sedimentation takes place on the drowned nver valleys Deltaic complexes are the first deposits to form above the preexlstmg lowstand sequence boundary These deposits are essentially retrogradatronal, bemg confined to topsets and clmofolms Coarse sediments are confined to the areas near the shore Not much sediment IS transported to the deeper basIn areas because there IS very httle subaenal eroSIOn Starved of sediments the submanne clastic deposits grow at a slow rate On the other hand, productIOn of carbonates, pelagic and hemIpelagIc sediments IS enhanced m the deep sea areas (Fig 9 21 B) These mark the top of the transgressIve systems tract The transgressIve systems tract pass distally mto a sectIOn charactenzed by a low rate of sedImentatIOn Glaucomtlc, phosphatIc shales, and also lutltes nch In orgamc content are produced under such conditIOn

Case 3 HIgh Sea-Level HIghstand Systems Tract HIghstand systems tracts are produced when the sea-level IS high The exact nature of depositIOnal sequence at this stage IS controlled by the relatIve rates of sea-level nse and sediment supply When the rate of sea-level nse IS low, the rate of creatlOn of accommodatIOn space IS balanced by the sedIment supply ThIS causes aggradation When the sea-level IS at ItS peak very lIttle new accommodatIOn space IS created and the sequence starts to prograde The succession showmg a sequence of aggradmg beds followed by a sequence of progradmg beds mark the hIghstand systems tract (FIg 9 21 C) The maximum floodmg surface IS marked by the plane demarcatmg the aggradmg and progradmg sequences Dunng transgression deposIts accumulate III the bas mal areas where accommodation space IS avaIlable The distal areas remalll starved of sednnents Frrmgrounds, hardgrounds, and extensive erosIOnal surfaces mark the sequence boundary III these places A rapid nse In sea-level may result III an abrupt change from shallow carbonate faCIes to pelagiC lImestones on both ramp and nmmed shelves

Application of Sequence Stratigraphy Sequence stratigraphIc concept advocates InterpretatIOn of sedImentary sequences III terms of relative changes III sea-level ThIs approach has opened up new dimensions to stratigraphic correlation and faCIes analYSIS IdentificatlOn of maximum floodmg surfaces allows large-scale correlation of the strattgraphlc umts Parasequences serve as keys to correlatIOn of the smaller UnIts In the MISSISSIPPI delta, for example, each delta lobe represents a parasequence (Fig 7 37), The first three of these lobes represent transgreSSIve systems tracts Others are highstand systems tracts (Kolb and van Loplk 1966) Sequence stratigraphIC technIque prefers genetic cntena for correlation Hence It IS more meamngful than the conventional techmque whIch uses purely descnptIve, lIthostratigraphIc cntena for correlation Observations on closely spaced outcrop and subcrop sectIOns (regional reflectIOn seIsmiC profIles followed by boreholes) are essential for sequence stratIgraphiC correlatIOn For temporal correlatIon, additIonal, bIO-stratlgraphlc or magnetostratIgraphiC lllformation IS necessary The sequence stratIgraphiC technIque, mltlally Illtended to deal With manne depOSits only, IS applIcable to sediments of flUVIal ongm also (see for example, Banerjee, et al

278


1996, Fnedman and Sanders 2000) This IS so because base level changes due to sea level fluctuatIOns, are eventually reflected In the fluvIal sequences When the base level IS lowered due to a fall In sea-level, a large part of the land surface IS exposed, allowmg extensIve development of fluvial channels Conversely, dunng phases of manne transgressIOn and sea level hlghstands, the amount of overbank accretion IS enhanced Palaeosols and sol1s beanng marks of oXidatIOn are produced on land dunng phases of manne lowstand HIghstand conditIOns lead to waterloggmg and orgamc growth All these features serve as markers for stratigraphIc correlation In the Upper Carbomferous BreathItt Group of eastern Kentucky stacked fluvIal bodIes are mClsed mto open manne and delta plam deposIts Systems tracts there could be IdentIfied from facIes associations, sedImentary body geometry and sequentIal position of the deposits (Aitken and Fhnt, 1995) In the TnassIc IVishak Formation of the Prudhoe Bay FIeld on the Northern slope of Alaska, recogmtlon of sequence boundanes, parasequence set boundanes and their Up-dIP eqUlvdl~nts aIded In long dIstance correlatIOn and classification (Emery and Mayers 1996, pp 130-133) While the IVishak study IS based on contInuous well logs, the techmque of field loggmg of the outcrops at centimetre scale was used for the Breathitt Group study The marker beds Identified on the logs were walked out along IndivIdual outcrops and correlated Examples of the apphcatlon of sequence stratigraphy In fluvial systems can be found m Wnght and Mamott (1993), Lmdsay, Holhday and Hulbert (1991) Posamentler, Allen and James (1992) applIed high-resolution sequence techmque for the study of Coulee Delta, Alberta In India, sequence stratIgraphic techmque has been applted III exploratIOn for petroleum m western offshore (Kutty and Agarwal, 1993) In the Bengal Basm, the stratigraphIc architecture durmg phases of Ohgocene lowstand has been conceptualtzed by sequence stratigraphIc techlllque (Das, Chakrabartl and Dutta, 1998) In the late Cretaceous Bagh carbonate sequences of the Narmada Valley Tnpathl and Lahm (2000) IdentIfied three hardground surfaces and mterpreted them as representatives of manne OSCIllatIOn events

10 BASIN ANALYSIS:

A

SYNTHESIS

INTRODUCTION Basm analysIs, the ultimate goal of a sedImentologIcal study, UIllavels the complete deposItIOnal hIstory of a sedImentary basIn This Involves syntheSIS of observatIOns on the framework of the basm, ItS sediment fill, and also the biota assoClated WIth the sedIments The stages of basin evolutlOn and the patterns of sediment dIspersal are also to be worked out In detatl In short, basin analysIs calls for complete knowledge of all aspects covenng the processes and products of sedimentation wlthm the sedImentary pnsm whIch constItutes a baSIn ThiS also demands a command over all the techmques and methods of sedImentology and related disciphnes on the part of the analyser U nttl about the fifties of the 20 th century basm analYSIS was a relatIvely SImple affarr It Involved, broadly speakmg, erectIOn of local stratIgraphic columns withm the sedImentary basm and correlatIon of these UnIts With the components of the conventional geosynclmal model whIch was popular at that tIme Basm analysIS today IS far more comphcated Mappmg the basement configuratlOn of a sedImentary basm now calls for intensIve applIcatIOn of vanous geophYSIcal technIques The geometry of the sedIment fill has to be worked out In three dImensIons usmg subsurface data, the nature of the fill has to be mterpreted In terms of faCies models, and the dIspersal patterns have to be determmed m quantItatIve terms, usmg the modern techlllques of palaeocurrent analYSIS To complete the story of basm evolutIon, one would hke to have radIOmetnc dates not only for the layers of sediments, but also for the mtruslves occurrmg withm the basm Fmally, today's students of sedImentology would hke to mterpret the stages of basm evolutIOn m terms of hthosphenc plate movements, followmg the currently popular model of plate tectolllcs Constant feedback of mformatIOn from field to laboratory and vIce versa IS essential The ultimate aIm IS to mterpret the products of sedlmentatlOn In terms of the processes Involved Co-ordmated effort on the part of specialIsts In vanous branches of earth sCience IS requrred to achieve thIS goal In most of the exploratory organIZatIOns thiS IS done by a team deSignated as 'the baSIn study group' A Simple flow chart for basm analYSIS IS presented In Table 10 1 The procedure to be followed IS diVIded mto three broad categones geologIcal, geophYSical and geochemIcal The sequence of analYSIS to be followed IS bnefly dIscussed below For detaIls of the analytIcal techmques the reader should refer to the chapters Cited The study of a virgin sedimentary baSIn commences With mappmg the depOSItional boundary and obtamlng as much Information dS possIble on the basm architecture 279

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and the llthlc fill Establlshrng llthostratigraphic Units, uncon fOrmitIeS etc PrepartIon of measured stratIgraphIc columns from surface and subsurface data Study of sedimentary structures, bIOta Thm sectIon petrography, modal, textural and heavy mmeral analYSIS, Clay mmeral studIes by XRD, SEM etc Palaeontological, bIOmetrIC and palynolOgical studies Determmatlon of major and trace elements, orgamc content, maturatlon etc RadIOmetnc datrng Magnetostratigraphy AeromagnetIC and gravIty surveys ReflectIOn seIsmIC survey, DSS DrIllrng and borehole loggrng

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SeIsmIC faCIes, and sequences Regional correlatIon Pattern of sedlIDentatlOn In relatIOn to tectonics Maturatton of sediments Basm evolullon With respect to geodynamICS and global sea level changes Hyrocar bon prospects

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The approach IS necessanly different for relatively small, Intracratontc basIns (e g pockets of Gondwana deposits WithIn the Indian craton) and large sedimentary basms of the type of the Bengal BasIn, which run across cratolllc or contInental margms and contmue on the oceanic crust While conventIOnal geological mappmg followed by exploratory dnllmg provides answers to most of the questions m the former case, a well co-ordmated geologlcal-geophys1cal approach IS essentIal for mvestlgatlOn of basms of the latter type ' An aeromagnetIc survey, which proVides the qUIckest method for obtammg the depth of the basIn -floor by detectmg the boundary between the magnetically susceptible, presedimentary, often metamorphosed basement, and the non-magnetic sedIments overlymg It, IS useful m detectmg the gross thIckness of sed1ments The overall drrectlOn of thlckemng and thInnmg can also be determIlled by the same method An aeromagnetIc survey IS generally followed by ground gravltymeter survey for obtammg mformation on denSIty anomahes In the subsurface The data obtamed are sometImes useful although the Inherent amblglllty In the InterpretatIOn of gravity data may not always prOVIde a umque solutIon to problems BASIN AND ITS LITHIC FILL Mapping, Petrography & Sedimentary Strcuctures

Geological mappmg of the lIthiC fill should be started Simultaneously WIth the aeromagnetIc survey The purpose of mappmg IS to establIsh the lItho-stratigraphic UllltS follOWIng the Code of StratIgraphic Nomenclature Mappmg begInS at the level of formatIOns The lower and the higher rankmg umts In the hIerarchy (member and group respectively) can be demarcated wlthm an area once the formatIOns have been establtshed The nature of unconformity below and wlthm the stratIgraphIc sequence should also be worked out m the course of mappmg (Chapter 9) Three-dimensional d1sposltlon of the rock umts wlthm a baSlll 1S represented by a vanety of composite maps prepared from measured stratigraphic sectIons In the absence of SUItable exposures, mformatIOn on rock umts IS obtallled either from boreholes or from twoway seIsmiC travel tImes from reflectmg mterfaces III the subsurface The reflectIOn tImes, converted to depths by means of mterval velOCitIes, are used for preparatIOn of structural maps, Isopachs, palaeogeologlc and paiaeotopographic maps Structural maps show rehef of the surface of mterest WIth respect to a datum which IS commonly the mean sea-level These are comparable to the topographIc maps prepared at the surface Paimspastic adjustments for foldmg, faultmg and regIOnal tIlt may be mcorporated mto these maps Isopach maps depict vanatIOn III thickness of the stratIgraphic umt of mterest ThiS vanatIon IS represented by contours, zero contour, mdicative of total merger of the upper and lower surfaces of the umt, defines the hmit of pmchout The shape of the umt thus defined often helps to deCIpher the ongm and depositIOnal environment of the concerned rock umt LithofacIes maps depict aenal vanatlOns In rock types (e g, sand, shale, carbonates), their proportIons, volumes and ratIOs (e g sand/shale, shale/carbonate) wlthm partIcular stratIgraphIC umts Isoltth maps show dlstnbutlOns as well as thickness vanatIOn of mdlvldual faCIes AerIal dlstnbutlons of cumulatIve thIckness of particular rock types IS shown by net rock-type maps

282


Havmg obtamed a broad pIcture of the basm archItecture and the overall nature of the basm fill, one should undertake a petrographIc study of the rock samples collected from the field Modal analysIs of sandstone samples by the pomt-countmg method proVIdes quantItatIve mformatIon on the mmeral proportlOns The sandstones are broadly clasSIfied mto aremtes and wackes on the basIs of matnx content The proportIons of quartz, fieldspars and rock fragments are then plotted on a QFRF dIagram for further subdIVIsion of the sandstone type (see FIg 3 3) ProportIOns of undulose, non-undulose, and polycrystalhne vanetles of quartz, plotted on the IndIana diamond dIagram (see FIg 24), gIve clues to the nature of the non-sedImentary source rock The heavy mmeral assemblage (see Table 2 2), degree of sortmg and roundmg (see FIg 49) of the clastiC partIcles prOVIde mSIght not only mto the pOSSIble provenance, but also mto the transportatIonal history of the sedIments Gram-SIze data may be utIhzed m a hmlted way for mterpretmg palaeoenvlronment and palaeohydrauhcs (Chapter 4) The carbonate rocks may be claSSified eIther by theIr depOSitIonal texture, as proposed by Dunham (1962), or by IdentificatlOn of the al1o- and orthochemlcal components under the mIcroscope (Folk 1959) The type of carbonate (caIClte/ aragonIte/ dolomite) IS deCIphered by staIllmg (Fnedman 1959) Replacement textures, If any, may also be IdentIfied at thIS stage IdentificatIon of mIlleralogy of the carbonates and the fine-gramed argIllaceous sediments, by more SophISticated methods, are undertaken at an advanced stage of mvestlgatlon The mmeralogical assemblage withm terngenous or chemIcal sedIments are often mdicative of the chemIcal condItIOns (Eh and pH) prevallmg at the site of depOSItIon (see FIg 3 19) In the second phase of fieldwork the pnmary sedImentary structures are studIed m de tall These can aId III stratIgtraphic smdles m many dIfferent ways Graded beddmg and npple marks, for example, proVide clues to the attItude (top and bottom) of the strata For thIS purpose, wave npples are of greater help than current rIpples Sharp crests and mdented troughs of wave npples can be easlly IdentIfied even when they occur m reversed pOSItion at the sole of beds (see FIg 5 24) In areas of strucmral complexIty, when the or1gIllal sequence IS mverted, the depOSItIOnal top and bottom of the sequence can be determmed from the wave npples The external morphology of a current npples IS of little help III the determInatIon of the deposltlOnal top of the sequence but the mternal cross-lammatton might be useful rn such a case (see FIg 5 24) Some of the common scour marks, such as flutes and grooves, have been traditIOnally used for determmatIOn of the depositIOnal top Such mterpretattons are based on the belIef that the groove and the flute marks are the counterparts of the ongmal casts formed as scours on the bed surface However, beds havmg POSItIve relIefs resemblmg flutes have been reported from the upper surface of non coheSIve sedimen t beds also (FrIedman and Sanders 1974) Thus the techmque of determmatlOn of the depOSitIOnal top from scour marks may not always prOVide dependable results

Palaeoslope and Palaeocurrent The mitial dip or slope of a basm-floor at the time of sedImentatIOn IS called the palaeoslope The term IS related to certam subaqueous depOSIts only EolIan depOSIts, controlled wholly by wmd movement, have very lIttle relationship WIth the slope of the depOSItional surface In flUVIal envIronment, where the flow IS essentlal1y

Basin Analysis: A Synthesis

283

unidirectional, palaeoslope can be obtained by averaging the cross-bedding foreset azimuths over an area. This is not necessarily so for marine or tidal environments where the palaeoslope may not be very consistent. Before attempting an interpretation of the palaeoslope, therefere, the nature of the depositing medium (water or wind) and the environment of sedimentation must be ascertained by criteria independent of palaeocurrent direction. Palaeocurrent analysis involves the study of ancient sediment dispersal patterns. Sediment properties indicating palaeocurrent are of two types: directional and scalar. Table 10.2 lists the more important of these properties. Following is a brief description of them. Table 10.2: Properties of Sediments Indicating Palaeocurrent (Modified after Pettijohn 1962)

Directional Properties

Planar Structures Cross-bedding Planar-tabular Trough-lenticular Low angle « 100) High angle (> 10°) Linear Structures On the base of bed ("sole markings") Striation and groove casts Flute casts Internal structures Clast lineation Fossil lineation Parting lineation On the top of bed Ripple marks Symmetrical Asymmetrical Rib-and-furrow Current crescents Complex Structures Convolute bedding Scalar Properties

Pebble and grain-size Pebble and grain roundness Thickness of bedding and cross-bedding

Directional Properties The directional elements in rocks indicate palaeocurrent direction at the point of observation. These directions are often allotted weights and treated as vectors. The directional elements again, may be of two types: planar and linear (see Table 10.2).

284


Planar DIrectIonal Elements

Cross beddmgs of vanous shapes and sizes are the commonest planar elements used for palaeocurrent analYSIS The dlfectlOn of mchnatlon of the cross beddmg foreset (the aZimuth of cross beddmg) mdlcates the dIrectIOn of sediment transport at the spot of observatIOn High and PIcard (1974) dIscussed the relIabIlIty of different types of cross beddmg for palaeocurrent mterpretatlOn For a planar-tabular cross-beddmg, palaeocurrent IS mdlcated by the dlrectlOn of mc1matlOn of the foreset Random measurement of dIPS on the flanks, m case of a trough type cross-beddmg however, might give mlsleadmg results Such observations must be corrected, takmg mto consIderation the plunge of the trough aXIS, to obtam the trough aXial aZimuth (Slmgerland and WIllIams 1979) When the area of study IS structurally tIlted or folded, a correctlOn has to be applIed to restore the bed contammg the dtrectIonal elements to Its ongmal posItion In the case of cross beddmg thiS IS done by rotatmg the plane of the cross-bedded Untt around the stnke of the true beddlllg through the dip angle of the beddmg Potter and Pettijohn (1977) have dIscussed thIS techlllque Empmcal studIes show that tIlt correction IS neglIgIble when the structural dip IS less than 25°, and also when the dIfference between the stnke dlrectlOns of beddmg and cross-beddmg IS small « 10°) In all other cases ttlt correctlOn IS necessary Computer programs for tilt correctIon are now avaIlable (Saha and Chakraborty 1990) ConclUSIOns on regIOnal palaeocurrent trends should be based, as far as pOSSIble, on a large number of observatIOns on cross-beddlllg of the same type, because they are lIkely to be mdicative of the same deposltlOnal envnonment Certam procedural problems are faced m the collectIOn and summation of dlrectlOnal data because the conventional techmque of anthmetic averagmg may not be applIcable m thiS work When the data are spread on either Side of the pomt of ongm, as m a compass dial, the anthmetlc mean may gIve absurd results (for example, the anthmetlc mean of 340 0 and 20° IS 180°) The usual techmque of obtammg standard deVIation can not also be used The procedure for obtammg the resultant dlrectlOn m such cases IS discussed m the appendIX to thiS chapter SummatIOn and representation of palaeocurrent dIrectIOns also pose speCIal problems The current directIOns are generally represented graphically, m the form of a cncular histogram ('rose diagram') (see Fig 10 10) CIrcular histograms plotted on hnear scales may gIVe a wrong ImpreSSIOn about the nature of dlstnbutIOn because the area of the 'pie' m such a case IS not proportIOnal to the frequency ('density') Circular histograms With areas proportional to denSity may be constructed by usmg the speCially deSigned, non-Imear frequency net deVised by Nemec (1988) When the geologIcal formatIOn under study contams a large number of dlfectlOnal features of a particular type, It IS necessary to know the mmlmum number of observatlOns that would give the mean drrectlon for the whole formation With a specified preCISion ThiS IS a sampimg problem The optImum hIerarchical samplmg procedure developed by Rao and Sengupta (1970, 1972) who reVIewed earher practIces, takes lUto account the speCIal requrrements of the radially distrIbuted data ThiS techillque answers the followmg questions 1) the mInimUm sample size reqUIred for estImatmg, With a desrred preCISIon the mean palaeocurrent directIOn of a formatIon and 2) the optimum allocation

Basin AnalysIs A SynthesIs

285

of samples between and wlthm the outcrops that would allow effiCIent samplmg at mmlmum cost A summary of this samplmg procedure IS gIVen m the appendLX to this chapter Lmear DIrectIOnal Elements

fupple marks are the commonest Imear structures used m palaeocurrent study Crest Imes of asymmetncal npples are usually normal to the directIOn of sedIment transport OnentatlOn of flute marks, grooves, skIp marks and other Similar features, generally preserved on the undersurfa'Ce of sediment layers as sole marks, also Indicate palaeocurrent Some other SImIlar features hke partmg ImeatIOn, onentatlOn of shell fragments may also be used m a simIlar way Current crescents splay out m the duectIOn of flow and often serve as good mdlcators of palaeocurrent (see Chapter 5) OnentatIOn and ImbncatIOn of sand grams and pebbles mdlcate palaeocurrent Pebbles are more commonly used for thiS purpose because they are easier to measure In such studies partIcular care should be taken while measunng onentatlOn of pebbles associated With a cross-bedded umt because pebble onentatIOns are likely to change, dependmg on therr locatIOns on topset, bottomset, or foreset of the cross-bedded umt Elongate pebbles tend to be transverse to the flow directIOn on the topset and bottomset of a crossbedded umt On the foreset they are longltudmal to the flow duectlon (Sengupta 1966, Johansson 1976) PresentatIon of DIrectIOnal Data

For convemence of presentatIOn the area of study 15 arbitranly dIvIded mto a number of sectors of equal size The resultant vector of all the duectIonal observatIOns wIthm each sector IS computed followmg the procedure given m the appendiX ThIS resultant directIOn IS plotted at the centre of the gnd The length of the arrow IS made proportIOnal to the vector magmtude m each case (Fig 10 I) While analysmg the trend of palaeocurrent directIOn m a large area It IS desIrable to emphaSIse the regIOnal trend of sediment transport Several technIques are avaIlable for thiS purpose In the relatIvely sImple, 'movmg average' method the area of study IS arbitrarIly divIded mto a number of sectors of equal SIze The resultant duectIon of all the data withm thIS area IS then moved systematIcally over the map WIth the help of a gnd The averages are then recomputed and plotted at the gnd coordmates (FIg 102) More SophIstIcated methods for Isolatmg the local from the regional are avaIlable, but the results obtamed from the sImple techmque of 'movmg average' compare well WIth those obtamed by these techmques (PettIJohn 1962) In the sophisticated techmque of trend surface analYSIS, different trends, Imear, quadratIC or higher are fitted to the observed data The 'reSIdual' values are then worked out from the difference between the computed and the observed values at each pomt (MIller 1956, Krumbem 1959) Computer programs for workmg out the trend surface are also available (Fox 1967)

Scalar Proertles Several rock properties lIke gram-size, gram roundness and bed thIckness show gradual vanatlOn m the dIrectIOn of sediment transport U nhke the duectlOnal features descnbed above none of these features prOVIde any clue to the flow dIrectIOn at the pOInt of

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288


observation. However, measurements of these properties at several locations spread over an area often provide clues to palaeocurrent. Variation in Size Progressive decrease of gram-sIze downstream IS a commonly observed phenomenon Deterrmnation of the mean Slze of a sediment populatIOn IS a difficult problem. As an alternative PettIjohn (1962) proposed the technique of measurement of the maXlInum pebble Size at a location. In actual practice ten largest pebbles are collected at each locahty and therr average values are plotted The flow drrectlOn IS gIven by the drrectlOn m which the mean pebble SIze decreases. In figure 10 3 for example, the flow IS from northeast to southwest, that lS, towards the direction of decrease of the mean pebble size Variation in Roundness

Pebble roundness generally increases towards the direction of sediment transportatlOn Systematic mapping of pebble roundness III an area, therefore, may provide a clue to palaeocurrent direction In the example shown III Fig 10.4 pebble roundness lllcreases towards southwest mdicatlllg sediment transportation towards that drrection The results are comparable to that obtained from measurements on pebble size (Fig. 10.3). . Vanation III pebble roundness is conspicuous m the imttal stage only, that IS, durmg the frrst few ktlometres of transportation. Thereafter, roundness becomes asymptotic to some partIcular value and shows lIttle change with further transportation (see Fig 4.10) In some cases, moreover, pebble roundness vanes only as a functIon of pebble SIze and may not be mdlcative of palaeocurrent drrectIOn at all (PettIjohn 1962) Scalar Properties Other than Size and Roundness

Apart from pebble size and roundness several other scalar propertIes lIke gradual vanatIOn III bed thIckness and cross-beddtng scale may provide clues to palaeocurrent drrectlOn. Both beddmg and cross-beddmg thickness show overall decrease towards the dlrectlOn of current flow. SystematIC mappmg of these properties, over a large area, takmg mto consIderation observatIons at as many locatIons as possible, IS needed for workmg out the palaeocurrent dIrectIon m these cases UnlIke directional propertIes, a stngle observatlOn on a scalar property prOVIdes no clue to palaeocurrent. In the case of subsurface data, however, scalar elements have a dlstlllct advantage over the drrectlOnal ones Palaeocurrent maps uttllzing scalar elements can be prepared from unonented core samples but onented cores are essential when directIOnal properties are Involved

Environmental Interpretation from Palaeocurrent Under favourable condItIOns, as dIscussed III the followmg sectIon, the pattern of palaeocurrent dIsperSIOn prOVIdes clues to the depOSItIOnal enVIronment In flUVial, deltaIC, and some turbIdIte sands the palaeocurrent IS generally ummodal, pomtmg downslope Eohan palaeocurrent IS unrelated to palaeoslope and may have any kllld of dIsperSIOn dependmg on the directIOn of wmd flow In the tIdal zones bImodal

289

Basin Analysis: A SynthesIs

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291

palaeocurrent is common. The shoreline current patterns may be of different types, depending on whether the currents are of marine or continental (fluvial) origin (Selley 1968, see Fig. 10.5). Palaeocurrent analysis may provide information on configuration of the basin margin, trend of the shoreline, direction of sediment source (provenance), pattern of sediment dispersal. This information, together with knowledge of palaeoslope and environment of deposition allows palaeogeographic reconstruction.

Depositional Environment It is not advisable to draw conclusion on depositional environment from a single line of evidence because a particular sedimentary structure or palaeocurrent pattern may occur in several environments in which similar hydraulic conditions prevail. An integrated facies model approach, taking into consideration the information drawn from various lines of evidence-lithofacies, sedimentary structure, texture, geometry of the sand bodies, nature of coarsening or fining-upward sequences - should be put together for drawing a meaningful conclusion. Most of the c.ommon sedimentary structures may originate in more than one depositional environment but particular combinations of sedimentary structures are often diagnostic of certain depositional conditions (see Chapter 7 fot details). The dispersion pattern of the palaeocurrent may, under certain circumstances, be indicative of a specific depositional environment (Selley 1968), For example, the palaeocurrent patterns within fluvial systems may be of two types-radiating and converging. The former is indicative of alluvial fans and the latter is characteristic of coal basins. The dispersion of current directions within piedmont fans and braided stream channels is naturally high. For meandering channels on other hand, the dispersion is particularly low (Fig. 10.5, a-d). The palaeocurrent patterns for deep-sea fans are unimodal, as in fluvial systems. Considered on a regional scale, however, two different patterns are possible. For smallscale turbidity currents, as in the outlet of submarine canyons or on slopes of delta lobes, the pattern is unimodal with small dispersion (Fig. 10.5 a). In turbidity-fllied troughs the general flow direction remains longitudinal and parallel to the trough axis' although sedimentation may take place laterally from the margins. This leads to an essentially unimodal pattern, with two subordinate modes lying nearly at right angles to the main flow direction (Fig. 10.5 f). In tidal flats bimodal and bipolar current directions, normal to the shoreline, are common (Fig. 10.5 h). Bipolarity develops due to landward flow of current during flood tide and flow in the opposite direction during ebb tide. A perpendicular arrangement of current directions may also be produced when fluvial currents flowing seaward alternate with longshore drifts (Fig. 10.5 i). In tidal inlets, on the other hand, the currents are unimodal, with small dispersion, being guided either by ebb or by flood (Fig. 10.5 e). Sea beaches may show a variety of palaeocurrent patterns, depending on whether the agencies responsible for sediment transportation are wholly marine or have both marine and tidal influence. Deltas produced by fluvial systems debouching into sea or lake may produce radiating palaeocurrent patterns (Fig. 10.5 g).

'ntroductlon to Sedimentology

292

a

c

b

¢

4 6 f

d

g

c>g
e

~ 4 h

¢

~

Fig. 105: Idealized palaeocurrent dispersion patterns In various depositional environments, (a) meandertng stream channels or turbidity flows (b) allUVial or submarine fans, (c) braided streams, (d) converging flUVial channels as In coal basins, (e) tidal Inlets, (I) turbidite filled troughs, (g) radiating distributaries In deltas, (h) tidal flats (I) longshore drifts superposed on flUVial currents (based on Selley 1968, Allen 1968a)

Subtle changes withm the sedimentary structures may also be used as mdlcators of vanatlon tn phYSIcal condltlOn withm a depositlOna1 environment Cross-strata sets, for example, are sometImes lllterrupted by slopmg surfaces called reactlvatzon surfaces (FIg 10 6B) These may be caused by changes In flow stages (a lowenng of water level followed by nsmg) of the deposltmg medIUm In the case of reversal of flow directIOn due to tIdal effect, the cross-strata above and below the reactivatIOn surface may dip m OpposIte dIrectIOns, gIVIng the ImpreSSIOn of herrmgbone structures (FIg 10 6 A) The lllterpretatlOn of depOSItIOnal enVIronment IS aided also by the logs of vertIcal stratIgraphIc columns, generally prepared dunng an advanced phase of field work These may be constructed eIther from the exposed sections or, preferably, from borehole data when these are avaIlable The field (or borehole) data may be utIlIzed for studymg the vertical vanatIOn m lIthofaCIes and also for prepanng maps Correlatlon of borehole data and preparatlOn of statlgraphic sectlOns also prOVIde clues to the three dImenSIOnal geometry of sand bodies, which are useful for envlronmental mterpretatlOn The chOIce of an appropnate datum IS Important for thIS purpose For example, the same sand body may look hke a dIstnbutary channel sand havmg a convex bottom or a sand bar WIth a convex top, dependmg on whether the upper or the lower surface IS used as the datum (Conybeare 1979) Wlrelme logs, run wlthm the boreholes, aId not only In regIOnal correlatIOn, but also In defintng the faCIes boundanes A bed havIllg a dIstmctlve log character may be used as a marker bed for the purpose of correlatIOn To some extent the log characters are mdlcatlve of depositlOna1 environments The depOSIts III low energy enVironments, as m lakes for example, are good markers but those In high energy enVironments, such as nvers, are 1m perSIstent No log pattern, however, IS untquely related to any particular depOSItional enVIronment For example, the gamma-ray or

Basin AnalysIs' A SynthesIs

293

S P patterns shown m FIg 9 13 can be found m any of the depOSitlOnal environments hsted m thIS figure A lme of eVidence, mdependent of the log pattern, must be used for choosmg the most probable depOSItIOnal enVIronment )

Flood tIde deposits

Ebb tIde deposits overlymg flood tIde deposits

Flow

===>

ReactlvatlOn Surface ~"..................... ,

.

Fig. 10.6: (A) Herringbone structures produced by reversal of current directions In tidal enVIronment (based on Fntz and Moore 1988) (8) Reactivation surface produced by changes In flow stage (after Harms et al 1975)

Under SUitable condItIOns dlpmeter logs may be also used for mterpretmg the deposItional enVIronment Four different dIP meter log patterns are generally recoglllzed (FIg 10 7) 1) the 'green pattern' showmg vertically constant dIPS, 2) the 'blue pattern' showmg downward decrease of dip, 3) the 'red pattern' showmg downward mcrease of dIP and 4) the 'random pattern' showmg no systematic vanatIOn of dip Although each of these four patterns can generate 1ll a number of deposItIonal envIronments, WIth some pnor mformatIOn on the type of enVIronment to be expected, one can make an mtelhgent guess about the type of environment mvolved In the case of an allUVial channel, for example, the thickest and the steepest cross-beds are expected near the basal scour ('the red pattern', see FIg 7 22), but m a progradmg delta the dIPS w1l1 decrease downwards ('the blue pattern', see Fig 9 IB) the Schlumberger (1970) dIpmeter manual dIscusses m detail the technIque of mterpretatlon Other uses of stratigraphIc logs lllclude analYSIS of the nature of CycllClty 1 e , IdentlficatlOn of auto and allocycles The vertically finmg-or coarsemng-upward cycles may also be worked out followlllg the techlllques suggested m Chapter 9 These also have Important beanngs on mterpretatIOn of depOSitIOnal environment

294


dIP o

o

o

~

o

o(")

o

o......

o

N

~.....

~

y '- ~ ~,;~e,'

"'?:i

1

A -~

A A

~

A

Fig. 10.7: TYPical dlpmeter log patterns

Palaeohydraulic Interpretations in fluvial Channels The dnnensIOns and types of sedimentary structures are mtlmately related to the hydrauhc conditIOns responsible for theIr ongm. Sedimentary structures may thus be used as clues to the geometry and hydraulIcs of anCIent flow channels. In one such attempt Bndge (1978, p. 725) used the followmg relatlOnshw vm = ~(8gYm' Sm) I f where Vm is the mean flow velocity for a cross-seeton at the position of mean depth (Ym), Sm IS longltudmal water-surface slope along the locus of mean depth (-channel centre line) of a curved channel, andjls the Darcy-Welsbach friction coeffiCient. ThiS factor IS eStimated to be 0 02 for honzontal lammatlOn m upper flow regime plane beds, 0.04 to 0.06 for large-scale trough cross-stratification in fully developed dunes and 0 03 for small-scale (npple) cross-stratIficatIOn, or tabular cross-stratification from dlmlmshed dunes (Bndge 1978, p.736) Allen (1968) obtamed the followmg empIrIcal relationship between height of subaqueous sand dune (H) and water depth (d) H = 0086 dl 19 ThiS relatIOnshIp, accordmg to Allen (1968, 1970), IS valId for sand dunes having heights rangmg between 0.1 and 10m A new set of relations connectmg water depth (d) and heIght (H) of sand ripples and small dunes (H = 001-009 m) was obtamed


295

expenmentally by Das and Sengupta (1998) H = 0 096 d 1 02 Field tests show that the error between computed and observed water depths obtamed by thIS equatIon IS small Moreover, the mfluence of gram-Size and flow veloCIty on H vs d relatIonship IS neglIgible when small sand npples (H < 20 cm) are Involved, and the flow ranges between 30 and 50 cmls Usmg a senes of hydraultc relattonshlps vahd for meandermg stream channels, Casshyap and Khan (1982) worked out the palaeohydraultcs of the PermIan streams responsIble for Gondwana sedimentatIon m the Bokaro basIn of IndIa Usmg SImIlar techntques, Gardner (1983) worked out the palaeohydrology and palaeomorphology of a Carbomferous meandenng stream In eastern Kentucky That such Interpretations can provide a consistent picture of the reglOnal palaeohydrauhc changes was shown by contounng the palaeohydrauhc parameters of the Karharban and the Barakar FormatIOns (PermIan) In the Gmdlh basIn. India (Sengupta etal 1988, see Fig 10 8) Startmg from Allen's (1968) equation, ~everal well-known hydraulIc relatIOnshIps were used 111 the followmg sequence for obtammg these results H = 0 086 d I 19 (Allen 1968) W= 42 dIll

= 109 Wi 0] * Lm = 1061 Q046*

Lm

(Allen 1968) (Leopold, Wolman and MIller 1964) (Carlston 1965)

Qrni A

(Schumm 1972) *These equatIOns, where the Ulllts are In f p s system, are used only as mtermedlate steps for estlmatmg the water dIscharge Leopold et al equatIon serves as an Intermediate step for estImatIOn of mean annual dIscharge For practIcal purposes mean annual flood (QmJ may be taken to be approxImately the same as bankfull dIscharge (Schumm 1972, p 103) V=

Explanation of NotatlOns

= mean dune heIght III metres d =bankfull depth In metres

H

= channel Width In metres Lm = meander wave length 111 feet Q = mean annual dIscharge In CUbIC feetl second A = channel cross-sectIOnal area In square metres W

V = flow veloCIty

1ll

metresl second

Qma = mean annual flood m cubiC mlsec = bankfull discharge (Q) This techmque of palaeohydrauhc mterpretatlon uses dune heIght as the startmg pomt for computatIOns The full height of the ongmal dune IS seldom preserved III geological record, hence a correctIOn IS often necessary JoplIng (1980) suggested a method of correctIOn on the assumptIon that about four fifths of the actual dune heights are preserved m the cross-beddmgs The true dune height may be estImated from thIS assumptIOn

.

50 ...

- -

-

,...

150_

Barakar FormatIon boundary

161

km

'-

110

'-

M[LE

\

'\...

....

- - - - Contours on srream dIscharge (m3/ s)

-

!

o

\ _ 144%6\-

/e

r5-54

',1)Il,

../

.

,526

- '" 50 ,;/1

/

,/

... ... .... .....

.... .... ....

,

...

...

.....

1'tb

"

o

Vi

,I

\ \

50

Q o

•

1089

1-

•

671

o

...

\

\

lO'N

24°

equations (from Sengupta et. al. 1988). With permission of Geological, Mining & Metallurgical SocIety of India

In the Glndlh basin worked out from magnitudes of bedforms uSing hydraulic

...

~.1124~"' ___ / I133~r... 2314 - -~150 • 700 .1398.

1476

~

Fig. 10.8: Barakar (Permian) stream discharge (m"/s)

~

'-

86° 115' E

'"

~

'<

IC

o 0'

j

-

c. 3

en

0'

:J

o

..o· c. r:

o

::3

:;

en


297

In the Gmdlh Gondwana basm, where channel geometry and palaeohydrauhc parameters were estimated by the above mentlOned method, the contours of channel depth, width and discharge run normal to the palaeo flow dlrectlOn obtamed from cross-beddmg aZimuths 'Moreover, the trends of width, depth and dIscharge parallel each other and all these parameters progreSSively mcrease m the downstream duectlOn, as m the present day flvers (see Leopold, Wolman and MIller 1964, p 240), thereby confrrmmg the dependablhty of the use of sedimentary structures m palaeohydrau1!c mterpretatlOns Diagenesis and Maturation Detailed laboratory mvestlgatIon of the core and surface samples IS recommended towards the end of the second phase of study While optIcal microscopy IS adequate for IdentlficatlOn of the framework mmerals, XRD or SEM techmques are needed for determmatlOn of the clay mmeral species Such studIes often prOVide clues to the diagenetic changes, WhICh the sediments have undergone The pattern of dIageneSIs m turn reflects the bunal, thermal and flUId crrculatlOn events to which the sedIments were subjected after depOSItiOn Under sUltable condItions the trend of diagenesIs can also be used as a clue to geodynamiCs and sedimentary basm evolutIon For example, precIpitatIOn of hIgh-temperature zeohte cements withm sandstones m the back-arc baSInS of the Western Pacific was mterpreted by Lee and Klem (1986) as the effect of upwellmg thermal convectIve clrculatlOn followed by a lower temperature downwelhng phase The zeohte cement was preserved because the sediments underwent slow bunal dunng the active nftmg phase of basm formatIon MaturatIOn of the orgamc matenal wlthm sediments dunng bUrIal m deep baSInS IS of particular mterest to petroleum geologIsts Dunng progressIve bunal of sedIments the orgamc components are converted to hqUld or gaseous hydrocarbons, dependmg on the pressure and temperature to which the sediment has been subjected LlqUld petroleum bemg preserved only wlthm a narrow range of pressure and temperature, called the 'hqUld wmdow', It IS of utmost Importance for petroleum geologIsts to obtam clues to the bunal history of the sediments PorOSity and permeabilIty of the reservou rocks, factors of Importance m prospectmg for natural hydrocarbon, are affected dunng dIageneSIs and cementatIOn ReductIOn of pOroSity, either due to compaction dunng bunal or due to chemIcal precIpltatlOn, takes place dunng dIagenesIs Although some Idea of the packmg denSity may be obtamed by countmg under a mIcroscope the number of gram-to-gram contacts III randomly cut rock sections, In SItu determIllatlOn of poroSity IS best done by usmg vanous types of wrre1me logs Combmed SP-reslstlVlty log, somc log, neutron log and denSity log are partIcularly helpful for In SItu determmatlon of pOroSity Somc log measures the time taken by acoustic waves to travel through one foot of rock formatIOn along the borehole Porosity ($s) IS then determmed by the formula $s = (M - Mm)/(!1tf - dtlmJ where !1tr::: transit time through the pore flUId, 6.tma ::: transit time through the rock matrlX, M::: mterval transit time (m modern loggmg manuals M IS often replaced by t)

298


The sonIc log, whIch responds to pnmary POroSIty IS vahd only for clean compact formatIOns with mtergranular pores filled by lIqUIds The presence of hydrocarbon In a lIqUId or gaseous state upsets measurements

Sediment Chemistry AnalysIs of the major and trace elements of the 'SedIment samples are undertaken at an advanced stage of basm analySIs RatIOS of varIOUS oXIdes, (Fe 20 3 + MgO)/TI02' Al2 0 3 1 SI0 2 , K 20/Na 2 0, A1 20/(CaO + NaO), may help deCIpher the tectomc settmg of the sedImentary basm (see Table 8 2) A SImIlar analysIs may also be attempted by plotting the framework mmeralogy of sandstones on Dlcktnson-Suczek dIagrams wIth QFL or QmFL as end members (see Fig 8 10) AnalysIs of the ImmobIle trace elements may also be used for workmg out the tectOnIC settings of the baSIn A progressIve mcrease 11l the proportIons of bght rare earth elements and decrease In U and Sc are expected from oceamc Island arcs to paSSIve margm settmgs passIng through contmenta11s1and arcs and actlVe contmental margms (Chapter 8)

Basin Evolution and Tectonics Most of the large sedimentary baSInS are defined by a senes of step faults bordenng the cratomc margIn, a sharp Increase In sedIment thIckness at the baslll centre, and hIghly deformed strata along the borderland WhIle the mobIle belt at the borderland can be studied by conventIonal geologIcal techmques, geophysical methods have to be applted for lllvestigatmg the first two features because they generally he concealed m the subsurface SubSIdence of the baSIn margIn concomItant WIth sedimentation allows a thIck pIle of sedIment to accumulate SImultaneously, a senes of growth faults develop m the areas of rapid subSIdence (see Dailly 1976 for detaIls) The beds located above these growth faults at shallower levels show flexures Another zone of faultmg generally marks the belt of adjustment between the stable shelf of the basm and the deeper trough of relatIvely rapid subSIdence ThIS was formerly deSignated as the' hmge zone' of the baSIn (Weeks 1952) In the Bengal BaSIn a conspICUOUS flexure In the Nummuhtlc (Eocene) lImestone honzon In the subsurface detected by seIsmIC survey, was deSIgnated as the 'hmge zone' (Sengupta 1966a) ThIS IS also the area where the SeiSmIC reflectIOns from the shallower sectIons above the 'hlllge' termmate sharply The zone IS also marked by an abrupt Increase m thIckness of sedIments downdlp (see FIg 822) ThIS zone, therefore, IS not a hInge m the tectonIC sense but possIbly a stratIgraphIc feature resultmg from mcrease m thIckness of the sedIment sectlOn and consequent compaction of layers In the lower part (see Conybeare 1979) Such an mterpretatIon suggests that the so-called hinge lme of the Bengal Basm IS In realIty the deposItIOnal hmit of the shelf sedIments On the other hand, later dIscovery of contmuatIon of the Nummuhtic hmestone Into the deeper honzons In the form of a flexure across the 'hmge lme' suggests that the lImestone hmge may be the shallower expressIon of a deep seated system of fault (Kalla et al 1992) The challenge of ba'Sm analYSIS ltes In searchlllg for the most plausIble answer from a host of such pOSSIble <;olutlOns A sedImentary basm occupIes only a thm layer In the outermost part of the earth's crust Thus our knowledge of a sedImentary basm remams Incomplete WIthout lllformatlOn of the lIthospherIc plate below the baSIn WhIle conventIOnal reflectIOn

299


seIsmic survey can provIde informatlOn on most of the major tectOniC features within the basm, deep seismic surveys (DSS) are reqUired for obtammg lllformatlOn from deeper parts of the lithosphere. The Importance of detection of thlllnmg of the crustal layer (e.g., the North Sea Graben) below a basm, or of a shear zone traversing the whole hthosphere plate (as in the Rotky Mountams) m tracmg the stages of evolutIon 0 0

0

II)

t:i

~

8

.q<

~

t::

'8

>-> ~

Vi Vl 'Vi

;3

~

~

~

Cl

~

t-.

~

t::

"
t::

'0.

t::

0 t.n

0 0

0

II) <')

M

C'l

0.. Vl

~

t:: ~

0

§

~

:-;::l

CIl

.

I

~

,

I . : ' . . JI' . . I .:', • '.' . ' I \ ,'. ... : . JI ~ '.- ..,. ~. --.

: ...

o

EstImated EustatIC

Sea·level 1 0 Chdnge 1 - - - - Lltho~phenc Flexure

-------?):

Rlftmg

Thermdl Lmear

r

10 30

50

~

Allegheny Orogeny

~

~

AcadIan Orogeny

Tacomc Orogeny

Basm fectol1lcS

SedImentatIon Rates (m/m y)

~ GrenvIlle RIftmg

t--;)

Coal

F·~··I ,. -

Crystallme Basement

D

Sandstone

~

Black Shale

fu1

Shale

~

L1mestone

I~·:::.f :::1

Volcamcs

-.-. ~

Sandy Shale

~

DolomIte

-:..,.--

Fig, 10.9: Correlation diagram of the Appalachian BaSin combining Information on sediment accumulation rates, sea-level Changes, orogenic events and stages of baSin evolution against time scale The ultimate aim of baSin analYSIS IS to prepare such correlation diagrams for sedimentary baSinS (redrawn after de V Klein 1987) With permission of ElseVier SCience Publishers B V

300


of a sedimentary basin, can hardly be overemphasized. A basin can be classified according to its genesis only when Its location with respect to the plate, the type of the underlying crust (continental or oceanic) and the major structural movements which have affected it are known (see Chapter 8 and Table 8.6). Well co-ordinated geological, geophysical and geocheimcal explorations are needed for this purpose. The ultimate aim of basin analysis should be to trace the history of evolution of a sedimentary basin in the context of global dynamics. 'Correlation of the local sections into a composite time scheme', as suggested by Dunbar and Rodgers (1957), is essential for this purpose. Measurement of the time span involved in basin evolution and sedimentation by radiometric, time-stratigraphic or other similar methods is needed. All these are represented in composite correlation diagrams of the type shown in Fig. 10.9. Recognition of stratigraphic 'sequences' and 'systems tracts' and correlation of the local events with global phenomena such as plate movements and sea-level changes are taken to be the goal of basin analysis today.

301

BasIn AnalysIs A SynthesIs

APPENDIX Techniques of Palaeocurrent Analysis. Treatment of Directional Data Computation of Resultant Directions The average palaeocurrent dIrectIOn wnhm an area IS represented by the resultant vector (y ) of all observatIons ThIs dIrectIOn IS computed as follows

Ungrouped Data Each duectlOnal observatlOn (cross beddmg aZImuth, npple onentatlOn, and the lIke) IS treated as a umt vector wIth components sm a l and cos a l The sums of smes and cosmes for the sample a l an are computed as n

n

V == Lcosa, W == Lsma, 1

1

The vector resultant (sample mean directlOn) IS gIVen as y= tan- I (WIV), If V> 0,

Y = 180 + tan I(WIV), If V
0

° °

W> Y = 270° + tan leW! V), If V= 0, and W <

Y IS undetermmed, If V = W = 0 Grouped Data

Suppose the data are grouped m the usual way mto k classes with x, denotIng the classmark of the Ith class and the Ith class has n, observatIOns for I == I, ,k k

k

Let V'== 2::n, cosx" and W'= 2::n, smx, ,-I

,=1

Then the mean duectlon I~ obtamed as m the case of ungrouped data, with Wand V replaced by W' and V'respectlvely

Variability A measure of vanablhty of the observatIOns wlthm an area IS gIven by I - R , where R == (W2 + V2)Y, , and R = R! n, n bemg the number of observatIOns, and R IS the concentratIOn In palaeocurrent maps concentratlon of observatIOns IS often represented by the notation L called consIStency ratIO where L = R! n x 100

Palaeocurrent Analysis Example The followmg cross-beddmg foreset aZimuths have been measured wlthm

the Barakar sandstone outcrops III a Gondwana coal basin Work out I) the mean palaeocurrent dIrectIon (vector resultant), 11) dlsperSlOn (consIstency ratIo) of these dIrectIOns The dIrectIOns measured are 5°,25°, 4SO, 48°, 50°,315°, 321°,322°,330°,332°, 335° 336°,337°,338°,342°,3520


302

Computation of Mean Flow Direction The data are grouped and sines and cosines are worked out in the following way. Table 10.3: Directional data of Barakar sandstone Interval

MIdpoint

0°_ 19° 20°_39° 40°_ 59° 300°-319° 320°-339°

95° 295° 495° 3095° 3295°

340°-359°

3495°

I I

3 I

8 2

cosC/J

W= n[Sm¢

V=n/Cos¢

017 049

099 087

0.99

076 -077 -050 -018

0.65 0.64

0.17 049 2.28 -0.77

0.86 0.98

-406 -036

6.89

-225

13.31

Sm

C/J

16

Total

Mean flow direction where W

nt

= In,.sin~;

IS

0.87 1.95 0.64 1.97

given by the resultant vector (Y) :::: tan-I WI V

V = Inl'cos~

y = tan-I (-2.25/13.31) =- 9.5949 (or 10°),360

0

_10° :::: 350 0

-JW2 + V2 = 13.50 Concentration is given by the consistency ratio, L =: Rln = 13.50 I 16 ~ 0.844 or 84.37%. A graphical representation (circular histogram) of these directional data is shown in Fig. 10.10.

R

=

1800 Fig. 10.10: Circular histogram ('rose diagram') of the directional data shown In table 10.3 The arrow Indicates dJrect:on of the resultant vector

303


Sampling oj Directional Data The procedure for samplmg of directional data (hke cross-beddmg aZImuths) IS summarized below. The reader may refer to Rao and Sengupta (1970, 1972) for further details.

Pilot Survey Collection of Samples: Before the actual sampling of the directional data like cross-

bedding azimuths is undertaken, it is necessary to conduct a pilot survey III the area of study with a small number of representative samples. An equal number of observations from each outcrop facilitates computation but this is not essential. Supposing the number of outcrops visited is n, and the number of observatIons taken from each of them is m, the total sample size N =n. m. Let
Analysis of Samples: 1. Sine and cosine values are computed for each direction r.t;} measured dunng the pilot survey. The length of resultant direction for each outcrop IS obtained as follows: R/2 ==

(I

cos

J=1

~/J)2 + (I sin
S/2

/=1

where Ri is the outcrop resultant for the ith outcrop, and m is the number of observations within each outcrop. 2. The overall resultant R for all outcrops is given by

R2 ==

(/~,cJ + (/~1

sJ

where n is the number of outcrops surveyed, and C, and S, are as defined above. 3. The analysis of variance (ANOVA) for the directional data is performed usmg Table 10.4. cD and ~ , the estImates for within outcrop and between outcrop concentration parameters respectively, are obtained by equating columns (4) and (5) of Table 10.4. Table 10.4: ANOVA Table for Angular Data { Rao and Sengupta, 1970, p. 536}

Source of VaYlatlons

df

S5

MS

(1)

(2)

(3)

(4)

Between outcrops ..

n-I

" LR,-R 1

CI R 1 -

E(MS) (5)

~(~ + ~)

R)/(n - 1)

n

N-n

Wlthm outcrops.

N-'iR,

(N - LR/)/(N - n)

2(0

1

Total .........

..

N-I

N-R

Note m is a weighted average of the m, ' S If m, = m for all outcrops then m =

III


304

4. An efficient sampling scheme should allow sampling at minimum cost. The optimum number of observations m* to be taken at an outcrop is obtained from the relation: m* = ~C1' ~ I ~. W . where C1 and C2 are the costs for reaching an outcrop, and taking an observation within the outcrop, respectively. An approximate idea of the relative cost (CII C2), say 10 : 1, or 20 : 1 is needed for this purpose. 5. The optimum number of outcrops to be sampled, n *, is given by the equation:

n* == K~[( 1/ p) + (1/ mw)] where Ko is the concentration required for a desired confidence level (1- u) and the semiangle of confidence (\)10)' Kois obtained from Table 10.5. The amount of precision required and the desired confidence level determine KO' Table 10.5: Value of Concentration Parameter KoRequired for YN to Attain a Given Precision (Rao and Sengupta, 1970, p. 537) 0/0 Semiangleof Confidence

5° ............... .

10° ............... .

20° ............... .

(I-a)

1('0

Confidence Level

Concentration Value Required

0.90 0.95

354.6292

0.99

870.6882

504.0610

0.90

88.7598

0.95 0.99

126.1125 217.9236

0.90 0.95 0.99

22.1772 31.5221 54.4496

Example: Sampling of cross-bedding azimuths from the Upper Kamthi member around Bhimaram (Bheemaram), India.

1. Fourteen outcrops of Upper Kamthi around Bhimaram were visited during the pilot survey. From each of these outcrops 10 observations on cross-bedding azimuths were recorded i.e. nu = 14, mu = 10 for Upper Kamthi. The total number of observations for the 14 outcrops Nu = 14 x 10 = 140. 2. The overall resultant based on all the 140 observations is Ru = (82.5159, -22.5615), with a vector length Ru

=

~(82.5l59)2 + (-22.5615)2

=85.5447. The

14

sum of the lengths of individual resultants is

~

RIU

=

91.8074.

1:1

3. Table 10.6 shows the ANOVA for the Upper Kamthi data. The value of F-statistic (F = 1.2593) being insignificant at 5% level (the tabulated critical value being 1.83 with 13 and 126 degrees of freedom), the variation between outcrops was not taken to be significant. In other words, all the Upper Kamthi outcrops were

305

Basin AnalysIs: A SynthesIs

taken to have the same mean dIrectIOn, whIch IS also the Upper Kamth1 Formatlon dIrectIOn The reasons for drawmg th1S conclusIon are d1scussed m detallm Rao and. Sengupta (1970, p 539) Table 10.6: ANOVA Table for Upper Kamthl Data (Rao and Sengupta, 1970, p. 539) Source

df

ss

MS

Between outcrops

13

62627

04817

Wlthm outcrops

126

481926

03825

Total

139

544553

E(MS)

1(1 +K 10) 1

"2

WU

F F == 1 2593

20lU

4. The estlmate of (0, say cOt ' IS obtamed by poo1mg the between and withm sum w~

= 2 5526 5 The equatlOn for K now reduces to K = N ffi~ Therefore, the number of observatlOns N wh1ch one should take m order to attam a sem1angle of say, 10° wIth 95% confidence (the correspondmg value of KO = 126 1125) 1S N = Kol ffi~ 126 112512 5526 = 4940 or roughly, 50 observations ThIS means that for Upper Kamthl one can estimate the mean d1rectIon to withm ± 10° wIth 95% confidence based on 50 observatIOns of the squares and the degrees of freedom Th1s gIves

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SUBJECT INDEX

(page numbers of t11ustratlons are III ItalIcs)

A

Arcuate delta 201 Arenaceous rocks 18-23 Aremte21, 23-25, 24 Argllhte 19 Anthmetlc mean 63 Arkose 25 Arran Cornstone 131 Ash faII 52 Assam-Arakan Basm 225-26,234 Atar Group 138 AtlantiC Ocean 212 Aulacogen 217, 231, 239-40 Authlgemc mmerals 54 Autocyc1es 5,256 AvulslOn 181, 183 Aztec Sandstone 173

AblatIOn 164 Absolute porOSI.ty 80 Abyssal plam 147·48,159 deposits 159·160 Acoustic Impedance 267 Aden Basm 225 Adsorbed layer 93 Aeromagnetic survey 281 Agglomerate 18, 52 Aggradation 248-49, 269-70 Aggradation 248,249 Aggradmg neomorphism 56 Agu1thus current 149 Albert Lake 227, 230 Algal mat 198 mound 36, 174 Allochemical 17 hmestone 61 Allocycles 5,256 AlIoformatlOn 247,256 Allogroup 5, 256 Allomember 256 Alluvial fan faCies 184-185 Alluvial fan 178, 180 Alpme-Hlma1ayan belt 207, 209 Alps 209 Amencan SOCIety for Testmg Matenals (A S TM) 63, 84 SIeve speCificatIOns 84 Anastomosmg nver faCies 187 stream 178,183-184 Andaman Basm 225-26 Sea 236-37 Angular unconformity 248,250 Antldunes 99-102 Appalachian Mts (basin) UA 15,207,299 Arc maSSifs 2 18

B Bababudhan Group 19 Back-arc 218 Backflow npples 103-04, 112 Backshore 192 Badanah, ArabIa 171 Bagh carbonate sequence Valley 278 Bahama Banks, USA 197 Bahamas 42 Bahram 171 Ball-and-pLllow structures 125 BaltiC Sea 174 Banda Arc 212 Banded (bedded) Hon formations 49, 51 Banded HematIte Jasper (BHJ) 51 Bangladesh 206, 244 Bankfull discharge (computatIOn) 295 Barakar FormatIOn 25-26, 189 sandstone 29, 295, 296, 30 I Barchan dune 169-70, 172-73 BarchanOld 169,170-71, 173

329

330


Barred (slUed) basIn 46, 49 Barren Measures 29 Baffler Island facies 190-191-92 Base level of erOSlOn 179,207 Baslll 207 analysIs 279-300 classificatIon 223, 225 III compressIOnal settmg 218 descriptIOn 223-244 evolution & tectOniCS 298-299-300 III extensional settmg 215 • III stnke-slIp and transform fault zone 219 study group 279 Bathyal zone 147-48 Bay of Bengal 162, 244 Beach berm 192 facies 190-191-92 Bed 246-47 sequence stratigraphic 273 stratigraphic code 117-118,246-47 thICkness vanatlOn 288 tractIOn load 95-97 Bedset 273 Bengal Baslll 206, 225-26,242-243·44,267-68, 278,298 delta 204·05,244 submanne (deep sea) fan 162,212, 236,244 BeOloffsubductlOn zone 2l3, 218 Benue Baslfl 217, 225, 231,233 River 239 trough (basm) 217, 231,233, 239-40 Bernoulli law 95 Bhaglrathl Group 206 Bhander Group 145 Bheemaram (Bhlmaram) 289, 290 Bhlma Basm 226 Bljalgarh Shale 31, 177 BljaWar rocks 49, 227 Blkamr·N agaur Basm 226,231 BIOdepOSitlOn 133 Blomlcnte 131 Blospante 131 Blosparrudlte 42 BlostratlficatlOn 133 BIostratigraphic UOlt 246 BloturbatlOn/blOturbated sediment 133,145, 191, 198,246 BIozone 247 Blrdfoot delta 201-02, 204-05 BIrds-eye structure 197 Black Sea 174,176 Bokaro BasJO 295 Bombay HIgh 58, 240

Borderland 207·08 Bouma sequence 157, 209 Bounce mark 124 Boundary layer 93 Boundstone 39,43 152-53 Box structures 130 Brahmaputra Valley 235 RIVer 97, 162, 206, 235 Braided stream 178-179-80 Brald-plam depOSIts/faCIes 18485 BreathItt Group 278 BreCCIa 18 Bnnk pomt (of sand npple) 103·04 Bunter Sandstone 173 Buntsandstem Sandstone 56, 188 Burst cycle 93

c Calc alkalme magma 234 Calcaremte 37, 39, 42 43 Calc1c vertlsol 131·32 Calc1te Compensation Depth (CeD) 159-160 Ca1ctlutIte 43 Calchthlte 23, 37 Calcrete 36,128-129,133, 198 Calcrete conglomerate 132 Cahche 36, 128·129 CahforOla Basm 225 Cahper log 261 Cambay Basm 226, 239-40 Canadian ShIeld 27, 163,224 CapacIty 99 Carbonate CompensatIOn Depth (CCO) 161 Carbonate sedIments on shelf 149·51, 154 Carbonates 33 chemIcal compositIon 34 chemIstry 33 faCies belts 151 mlcnte 38,43 petrography/ classificatIOn 37·43 spante 38, 43 Carboniferous cyclothem of Yorkshlfe 256 Cataciastlc conglomerate 19-20 Cathodo lumillescence (CL) 5, 55 Catskill Delta 252 Cave pearl 36 Cement 55, 61 Chalk 159 Chamuna Limestone 145 Chandarpur FormatIOn 24, 28 Chandlpur Flat 195, 199·200 Chandpur Formation 28 Channel lag depOSit 187

331

Subject Index

Channel sand stackmg 188 Chart 129 Chattermarks 164 Chattisgarh Basm 117, 126, 134,226 Group 145 Chevron mark 124 Chezy equatlOn/fnctIon coeffiCIent 94 Chllla Basm 225 Chitradurga SChIst Belt 222-24 Chorhat Sandstone 146 ChronostratIgraphIc umts 24b-48 Chute cut off 182-83, 187 deposits 187 CIrCular hIstogram (rose dIagram) 302 ClastIC SIlls and dykes 127 texture 17, 61 Clay deep-manne (red) clay 31 mmerals 32, 55 clay stone 30 ClImbmg npple lammatIOn 200 trans latent strata 173 ClInoform 270 Chnothem 149,152 Coarsemng-upwardsequence 160,178,206,249, 261,264 Code of StratIgraphIc Nomenclature 246, 271, 281 Collena 135, 138 CollUVIUm 10 Colorado estuanne channel 205 Colorado Plateau 215 Combmed flows 116 Competency 99 Composite basllls 240-244 ConcretIOns 128 Conglomerates 18-20 ConsIstency ratlo 30 I Contlllenta11ce sheet 164-65 Contlllental margllls 213 nse 148, 155,212 shelf 147-48 slope 155 terrace 212 Contlllent-contment co1hslOn 234 Contlllent-ocean colliSion 234 ContlllUlty equatIOn 88 Contorted structures 126 Contnbutlllgnet 178-79 Convergmg plate boundanes 213-14 Convolute beddmg 124-26 Cornstone 129,132 CorrelatlOn'259-267

dIagram 299-300 of flUVIal channel sands 265 techmques 260-267 CorrelatJve conformIty 271 Coulee Delta 278 Craton 208 Crevasse splay 181-83, 206 depOSits 187, 202 CntIca1 erOSIOn velOCity 92 Cntical flow 89 Cross-Iamlllation 108, hernng bone 198,293 Cross-stratIficatIOn 102-112, 109-11,114, 189 beddmg 108-112, 189,284 Cryptozoon 33, 135,138 -Cuddapah BaSIn 226,231,234 Cumulative curve 72, 74 Current crescent 121-122, 285 Current Itneatlon 118 Cuspate delta 20] CycliCIty 256-259

D Dakota Sandstone 131 Danaktl depreSSIOn 229 Darcy equatlOn/fnctlOn factor 94 Darcy Imllhdarcy 81 Death Valley 48 Debns flows 155·56 Deep SeismIC Soundlllg (DSS) 234,299 Deeper Bengal Baslll 243 Deep-sea fan 155-158, 162 DeflatIOn 168 Deformed beddlllgs 124-125-126-27 Delta 179 type baslll 239 DeltaIC enVIronment 200-206 DenSity (turbidIty) current 27,88, 149,155 DenSIty log 261 DeposltJonal enVIronment (lllterpretatJon) 291293 sequence 271 top and bottom 282 DeSIccatIOn cracks and polygons 118,175, 197. 198 DiagenesIs 17, 53-58 carbonate sediments of 56 SIlIceous sediments of 54 DIamIcts 19-20, 164, 166 DIamond dIagram 14-15 Dlastem 251 Diatom 48


332

DlIatance 155 Dlpmeter log 180,249, 261,264, 293-94 DirectIonal data samplIng 303-305 treatment 301 properties 283-285, 288 presentatIOn 285 Dlsang thrust 235 Dlsconformlty 248,250 Dish structures 127 Dlstnbutmg net 178-79 Dlvergmg plate boundanes 213-14 Dolmlte/dolomltlzatlOn 41-45,46 Dolo,tone 41,45 Dome 169 Dott's scheme (of sandstone clasSIficatIOn) 2223,28 Do\\nlap 270 Downwarp basin 223-225 Draa 169 Drag fOlce 92 Drams Fjord 174 Dnft (glacial) 164 Dnft potentIal 169 Dropstones 166 DrumlIn 168 Dual hierarchy In stratIgraphy 246 Dune 99-101, 105-06, 168 182 cia;slficatlOn 106 O",arkcs\\ar RlVer 76 O",yka, South AfrIca 168

E East AfrIcan RIft System 230 East Indlc, 212 Eastern Hlmalclya ::35 EffectIve poro':,)ty 80 Eh-pH control In sedimentation 50 cl Mreltl Group 138 Electron probe mIcro analY~ls (EPMA) 5 Endlchnta/ExIchnta 141 EndogenetIc rocks 17 [mlronmental Influence 246 Emlronmenta1 interpretation (from palaeocurrent) 288-291 [OgCnctlC changes 54 Eolian dunes 169, 192 cnvlronmen t 168-173 [phemeral stream 178 fplclastlc conglomerate 18,20 [plgenetlc dolostone 45 f:plffiatnx 25 l:pnelIcf (I' plchnla) 141

EpI~odle depmlts 257 EqUIvalent ;and thlckne;s 169 E,ker 164, 165,168 Estuary 200. 201 EthIOpIan nft 217 Eugeosynclme 208 Euxmlc baSIn 174, 176 EvaporatIve pumpmg 45 Evaponte 46-48, 178, 198 Exogenetlc rocks 17 EXXON LaboratOrIes 269

F FabrIC hypldlOtOPIC 45 IdlOtOPIC 45 xcnotoplc 45 FaCIes (flUVIatIle) aSSoClatlOn In space and tIme 190 FacIes 147 aSSOCIatIOns (sequence stratIgraphIc) 274 change 3 model 147 FaIled arm (aulacogen) 239, 24) nft 231,241 Fecal j.Jellet 38 FelSIC magma 234 Fenestral pores 198 porOSIty/structures 197-98 FerrandolOffilte 42 Ferncrete 130 Ferruginous concretIons 129-130 nodules 129-130 Fmmg-upwardsequence 160,182,185,187,192, 249,256, 261,264 FIre Island Inlet, New York 193 Flaser beddmg 108,197.200 Flonda 42 Flo\\< 89 regIme 90, 99 regIme. lower 90.99-100 regIme, transitIOnal 99- I 00 regIme, upper 90 99-100 separatIon 103-04 Flo\\ sheet for baSIn analysI~ 280 FIUldIsed flo\\s 156 F1Uldity 88 Flute marks 121 123,285 FlUVIal Cacles models 184 FlUVial envIronment 178-190 sequence 190 Flysch 209 Folk & Ward formula 8::;,85-86

333

Subject Index

Fondathcm 149 152 Fore (outer) arc 218 Foreland 208 ba,ms 234-235 spar 234 Foreshore 192 FormatlOn 246-48 Frequency polygon 72 Fritsch sieve shaker 64 Frost heavwg 8 Froude Number 89, 99

G Gabon Basm 225,236, 238 Gamma-ray log 261 263, 266 Ganga Basm 226,228, 234, 242 Ganga River 185 206, 210 Ganga-Brahmaputra Detla 203-04, 236 Garo Hllls 241 Geanuclme 208 Geode 129 Geological time UOlts 246 mappmg 281 time-estimatIOn 245 246 Geometnc mean 63 Geosol247 Geosync1mal concept 207-213 Geosynchnc 3, 149,207 classificatIOn 212 Geysente 133 Ghadames, Libya 171 GhatsIla 209 Gilbert-type delta 108,177,201-202,270 Gmdlh BaSin 286, 295-96 GlaCIal environment 164-168 GlaCIated pebbles 22 GlaClo-lacustnne faCIes 166 Glaebules 129 Global sea level fluctuation 256-57 Godavan Valley 26, 188 Gog Quartz! te 163 Gomtl River 189 Gondwanaland 241 Graded bed 118-19,209 rhythmites I 18 Grade-scale 62 Gram contact 70 Gram-fall lammatlon 172 Gram flo\\s 156 Gram-size analYSIS & mtcrplctatlOn 67 classificatIOn & nomenclature 63

dlscnmmatlOn 68, 73 measurement 6267 statistICal parameters 83-86 tcchmques of analYSIS 64, 82, 85 vanatlOn 288 Gram packmg 61 Gram-falllammatlOn 172 Gramstone 38,43,153 Gram-to-gram relationship 79 Grand Banks earthquake 157 Gravelly allUVial faCIes 184 Gravity waves 88 Gravltymeter survey 28 I Graywacke 28-29 Great BItter Lake 48 Greenstone 27, 209 Gronmgen sedimentation balance 67 Groove marks 124,285 Group 246, 247 Growth fault 202,203 Guano 48 Gulf of Gumea 231 GytIJa 174

H Hairpm vortex 91 Hard ground 198,278 Harmomc mean 63 Harz Mountam 27 Heavy mmeral assemblage 260 Heavy mmerals 13, 16, 23 Hedleberg Group 257 Hemapelaglc depOSits 161,276 Hernngbone structure 198, 292-93 H!ghstand systems tract 276-77 HImalayas 162,164 HInge zone 241, 243, 298 Histogram 72 HJulstrom diagram 92 Hoyt Limestone 133 Hummocky cross-stratificatIOn Imega npples 115-116,149,160,163-64, 166,177-78,191 HydraulIc Jump 90 radIUS 88 Hydrocarbon 297 Hydrological cycle 8 Hydroplastlc or hqUlfactlon layers 127 HYPldlotoplc fabnc 45 HyporelJef(Hypochma) 141

I IchnofaCIes 142


334

ichnology/ichnofossIls 5, 140-146 IdlotoplC fabnc 45 19mmbnte 52 Illltlza tlOn 55 Imbncatlon 122, 285 Imra stone 129 Inchon Bay, Korea 195 Incluslve graphlc measures (Folk & Ward) 68, 85, 86 Index of packmg 80 Indian Ocean 236-37, 242 floor 162 IndoneSia 237 IndoneSian Arc 212 InductIOn logging 264 Indus River 210 submanne fan 162 Inferential units 246 Intenor baSinS 231-234 InterpenetratlVe convolutIOns 127 Intraclast 37 IntraformatlOnal conglomerate 20 IntramlcrudHe 42 Intraplate basms 213 settmgs 213 Intraspante 41,42 Inverse gradmg 118 Iral, Indla 168 Iron Ore Group 58 Iron-beanng sediments 49-52 Ironstone Shale FormatlOn 25, 51 Iron~tones 49 Irrawaddy Delta 237 Isohth map 281 Isopach map 281 Isostatic adjustment 215 IVlshak FormatIOn 278

J Jalsalmer BaSin 231 Jammu Limestone 138 Japan Trench 212 Java trench 212 Jhana Basm 225 226-27, 232 Coalfield (basm) 24, 31

K Kalmur FormatIon 163 KamIlano, Austraha 168 Kamthl Formation 188 289 Kankar 36 Kansapathar FormatlOn 117 KaoilmtIZatlOn 55

Karharban formatIOn 295 Karst 36 Kathlawar horst 240 Kelvm-Helmholtz mstabllIty 102 Kerguelen hotspot 244 Keshavapura Graywacke 222-24 Keystone vugs 198 Klmberhte-carbonatltes 234 Kmematlc VlSCOSlty 89 Klaralven Rlver 181 Kota Llmestone 139 Krol FormatlOn 34,49 Krumbem-Garrels diagram 49-50 Kuge!sandstem 129 Kupferschlefen, West Europe 177 Kutch Basm 226, 239-40 Kutch-Cambay Basm 225-26

L Lacustrme environment 174-178 Lagoonal faCies 193 Lamayuru FormatIOn 164 Lamma273 Lammar flow 89,90 sub layer 93 Lammaset 273 LammOld fenestrae 197 LandslIdes 10 Laser scatter 64 Lateral (stratIgraphic) relatlOnshlp 251-254 accretIOn 185, 187, 248, 249 LatentlsatlOn 54 Law of faCies succeSSIOn 3, 251,253 Law of ongma1 coutmUity 2, Law of ongmal honzontahty 2, Law of superpositIOn 2, 189,245 Lee face 102-03 Lenticular beddmg 108, 197 Levee (levee) 181-82, 189-90 Limestone 34 allochemlca161 claSSificatIOn (Folk's scheme 39, Dunham's scheme 41) formmg enVHonment 35-36, 44 Fawn llmestone 34 hthographlc 34 nummuhtlc 241 oohhc 30 orthochemlcal 61 Rhotas Llmestone 34 scheme for descnptlon 60 trace elements In 34-35, 298 Lmear (self) dune 169,170, 171, 172

335

Subject Index

Liquid wmdow 297 Lithic fill ofbasm 281 LIthodeme 247 Lithofacies map 281 Lithographic stone 40 Llthosome 251 Llthosphenc plate 2 13 LithostratIgraphic umts 246 LJunggren-Sundborg dlagram 98 Load casts 125 Lobate delta 201 Loess 166 Lofentes 197 Log-hyperbohc dlstnbutlOn 71, 182 Log-normahty 70-71, 182 Log-probablhty plots 70, 74 Loulslana coast 202 Lowstand systems tract 274-275-276 Lutaceous rocks 18, 30-32

M Magnetlc stripes 213 Magnetostratlgraphy 245-246 MaliIao Flat, Talwan 195 Malen Formation 132 Manganese nodules 128, 159 Manmng equation! roughness coefficient 94 Manne environment 147-164 Manne glacIalfacles 167 Manne transgression 250 Markanda Rlver 185 Markanda termmal fan 186 MarkOVian property 259 Marl 174-75 Marlstone 36 Mass wastmg 10 transport 155 Matenal umts 247-48 MatriX 18,21,25,61 MaturatIOn 297 Mean annual flood (computatIOn) 295 Meandenng stream 178179-81, 189,295 faCies 185, 187 Mechamcal disaggregation 9 Median fall dlameter 99-1 00 Mediterranean Sea 176 Member 246, 247 Mesogenetlc changes 54 55 Meteontlc conglomerate 19-20 Michigan baSIn, USA 231 Mlcrospars 58 Mmeral stabllIty 10 MlOgeosynchnal furrow 208

MlOgeosynchne 208 MISSISSIPPI River and Delta 202-203 204 205, 277 Mixed environment 190 200 Modal analYSIS 282 MoenkopI FormatIOn 188 Molasse 209 Moment measures 67, 86 87 Montana USA 267 Movmg average (of duectIonal data) 285,287 Mt Manon Formation 29 Mud cracks 118120 flows 10, 11 8 Mud stone 30, 40

N Naga thrust belt 235 Nahan Group 25 Nahorkatlya Oilfield 235,26465 Nan 36 Narmada BaSIn 23940 225 NavajO Sandstone 173 Neck cut-off 18283 Neomorphlsm 56-58 Neptumsts 3 Nentlc zone 147 Neutron log 261 Nevada 156 Nlger Basm/Delta 22526,233,239 Rlver 239 Nlle Delta 203-04 NlmarFormatlOn 163 Nmety East Rldge 242 N oamundl BaSIn 51 Nodules 128, 129-130 N on-clastiC texture 17 Non-conformity 248,250 North Amencan stratigraphiC code 247 (table) North Sea 195,215,299 North Sea Graben 299 Nuee ardente 52 NummuhtIc(Eocene) Llrnestone241, 24344 298

o Observable UllltS 246 Offlap 250, 270 Old Red Sandstone 131,185 187 Ohgomlcts 19-20 Ohstohth 164 Ohstostrome 161 Oncohth1135,138 Onlap (over lap) 250 OolIte (ooids) 37, 43 Oomlcrudlte 42


336

Oospante 40, 42 Open channel flows 88 OphIOlItIC melange 164 Orgamc evolutlOn 246 Orthochemical 17 hmestone61 Orthogeosynchne 212 Orthomatnx 25 OscillatIOn npples 114-15 Overgrowth 13, 54 Overlappmg cycles 10 delta complex 255 Oxbow lake 181-83

p Packmg80 Packstone 39, 43, 152-53, 198 Palaeocurrent 4, 282-291 analysIs, technIque 301-305 duection 286, dIspersal patterns 292 PalaeoenVIronment 282 Palaeohydrauhc mterpretatlOn 294-97 PalaeomagnetIc studIes 213 Palaeoslope 282-283 Palaudal sedImentatIOn 203 Palaeohydrauhcs 282 Paleohydrauhc mterpretatIOn 294295, 296,297 Paleosol 132, 188,278 Panchet FormatIOn 29 Papagham Group 40 Para Lava 31 Parabohc dune 169 Paraconformity 249 Parageosynchne 212 ParalIc sequence 209 Parasequence 271, 272, 274 Partmg ImeatIon 118, 285 Peat 174 Pedogemc calcrete 129 PelagIC deposIts 161 Pellets (pelOIds) 37, 43 PelmICflte 42 Pelspante 42 Penecontemporaneous deformatIon structures 124 PennsylvanIan cyclothem of IIlmOIs 256 Percentage estImatIOn 59 Perenmal stream 178 Penod 246,247 PermeabIlIty 61, 79, 81 Permeameter 81 PersIan Gulf 154 PetrographIC techmques 59 60

Petroleum exploratIon 81, 297 Petrornlcts (polymlcts) 19 PhI scale 62 PhosphorItes 48-49 Photon correlatIon spectroscopy 64 PhotosynthesIs 33, 51 PhreatIc zone 56-57, 129 PhyllarenIte 23 PIllow lava 208-209 Pmchout250 PisolIte 36, 132 Planar dIrectIOnal propertIes 284 Plate boundanes 214 Plate tectomcs 213 concept 213-215 Playa lakes 42 Pomt bar 181, 185,189-90 POIse 88 Polanty chronozone 247 PolycyclIc sedIments 13 Pool 182 PorcellanIte 53 PorOSIty 61, 80-81 PorOSIty, In SItu determmatIon 297 Powers' chart 75 Prandtl-Karman velOCIty dIstrIbutIOn 94-95 Pranhlta-Godavan Valley 145 Pressure solutIOn 55, 130 PnncIple of umformitanamsm 3,212 Probablhty plots 68, 74 Prod mark 124 ProgradatIon 248, 270 Protomatnx 25 Provenance 13-14, 16 PseudomatrIx 25 Pseudonodules 125 PseudostratificatlOn 106-07 Pull-apartbasm 236,238-39 Punctuated Aggradanonal Cycles (PAC) 258 259 PyroclastIc conglomerate 19-20 PyroclastIc debns 52

Q Quartz aremte 23-24 Quartz wacke 25

R RadIOactIVIty 245 RadIOlarIa 48 RadIOmetnc method of datmg 245 Ramdrop Impnnts 119 Ra)mahal hIlls 241 Ra)mahal volcamcs 241

Subject Index

Ramganj Coalfield (basm) 24, 49, 51 Ramganj FormatIOn 24-25, 29 Rann of Kutch 48, 200 Rapid (supercntlcal) flow 89 90 Reactivation surface 292 93 RecesSIOn marks 118-19 Red Sea 176,229 Reducing (euxmlC) basin 174-176 ReefOidal complex 163 Reflux theory 47 RegIOnal palaeocurrnt trend, technique of interpretation 287 Regolith 10,26, 132 RegressiOn 270 Residence time 10 ReslstlVlty log 260, 262 Retrogradation 269-70 ReverSing-crest n pples 114, 115 Rewa sandstone 23 24 Reynolds Number 89 RhlzocretlOn 129,132 Rhone Delta 203 Ridge-and-swale 181 82, RIffle 182 Rift basm 227, 231 Rill marks 118 120 Ripple classlfic tlOn 104 106 dnft cross-lammatlOn 106-07, 199 height 102-03 mdex 102, 113 mterfenng 197 marks 103,285 morphology 103 tlmmg-fork shaped 195 Roches moutonnee 164, 168 Rock stratIgraphic umts 246 Rockfall 10, 155 Rockshde 155 Rocky Mountains 163, 177,215,218,223,299 RollabllIty 78 Rolhng-gr~lIn npples 114-15 Rose diagram 284 Rosettes 128, 154 Roundness 72-75-76 vanatlOn 76, 288, 290 vanatlOn of pebbles 76, 290 Rubble 18 Rudaceous rocks 18-20 Runnel 191

s Saamch Inlet 174

Sabkha 36 42,46,48, 154 Sal t marsh 1 97 SaltatiOn load 9596, 168 Samp1mg problem 284 Sand flow lamination I I 3, 172-73 lobes 124 nbbons 150 npples 99- 103-105 npple classificatIOn 104-106 waves 96, 105 Sandar 166 Sandstones 19 arkose/feldspathlc 25-26 chemical compositIOn 27-29 claSSificatIOn 21-23 graywacke 27-29 modal analYSIS 24 orthoquartzIte (quartz arelllte) 23-25 quartz wacke 25 scheme for descnptIon 59 60 subarkose 26 Sandy allUVial faCies 185 Sandy braided stream 185 Sapropel 176 Saloldl ArabIan Platform 223 Scalar properties 283, 285, 288 Schenck-Muller scheme 246 Scour and fill 108 Scour marks 121 Seatearth 188 Sediment chemistry 298 dispersal pattern 283, 292 transportatIOn 95-g9 Sedimentary baSinS 207, 215-219 Seepage reflux hypotheSIS 45 SeismiC correlation 266,268 SeismiC stratigraphy 5,266-267 SEM picture 63·64 Sequence stratigraphy 5, 267-278 appitcanon 277-278 history 267-268 Settlmg techmque 65-67, 70 Shale 30 BIJaJgarh 31, 1 77 black 31,176 chemical COmpOSItiOn 31 classification 31 deposltlonal environment 32 Shape 76-78 Shark Bay 137-38,198 Shear boundary 213 Sheet sand 169 Shields' curve 92-93

337

338


Shoestring sand 187 Shoreface 191 Shrinkage cracks 118 Sieve texture 184-185 Sieving \echniqueo~-o5, 1~, ~1 Silicic acid 9 Siliciclastic tidal flat 195,199 Silled basin 46, 49 Siltstone 30 Similarity theorem 94 Simla series 209 Simons-Richardson-Nordin diagram 100 Siwalik Basin 210 Group 132, 209 lithological columns 2J 1 Size variation (of sand grains & pebbles) 288-89 Skeletal (fossils) 37 Slump structures 127 Smooth and rough boundary 93 Sale marks 124-25 Sonic log 261, 263 Sorting 60 South Karanpura Coalfield 189 Southard-Boguchwal diagram 101 Spar 37 Sphericity 77-78 Spherulite 128 Spider diagr~m 259 Spontaneous potential (SF) 260-62, 266 St. Michel Tidal Flat 195-196 Stabihty series 12 Stable shdf (Bengal Basin.) 243 Stalactite 36 Stalagmite 36 Star (pyramoidaI) dune 169,171-72 Stokes' equation 65 Stoss face 102-03 , StratIgraphic correlation 138 Stratigraphy-classical concePts 245 Stream discharge 296 St;ream power 99-100 Strike-slip fault 219, u Stromatolites 133, 134, /35, '136. 137·38-40, 1'54, 175, 197-98 abundance 138 classlficanon 134-137 Structural map 281 Stylolite 55, 130-131 Subarkose 25 SubductlOn 213, 218 basins 236 SublevatlOn 251 Submarine canyon 155,162, 202 fan 158, 162,212

Submersibles 5 Sumatra BaSIn 237 SummIt point (of sand ripple) 102-03 Sunda arc 212 ~upergroup

'})\6-41

Surface creep 168 Suspension load 95-96-97, 168 Swaley cross-stranfication 116-17 Swash marks 120 Swatch of No Ground 162 Sydney Basin 132 SynaeresIs cracks 118-119 Syngenetic dolostone 45 System 246-47 Systems tract 271

T Tadpole plot 264 Talchrr FormatIOn/deposIts 166-161-68 Talchir glacial sequence 167 Talus creep 10 TanganYika Lake 227, 230 Taoudenni BaSIn 138 Techniques of sedimentary petrography 59-60 TectOnIC control of sandstone compositIOn 219221·224. Telegenetic changes 54 Tempesites 118 Temporal units 247-48 Tepee 129, 198 Tephra 161 Terminal fall velocities 69 TermInal fan, Markanda 186 Terrigenous 17 Thalweg 181 Thermal doming 216 heating and cooling 231 Thermocline 175 Thin-skinned thrusting 216, 218 Thrombolite 135 Tid
339

Subject Index

Trace fossils 140-41, 143, 145-146 classlficatlOn 141-142 occurrences 144-146 use m envlfonmental mterpretatIon 142-144 use m stratlgraphy 142 Tranquil (subcntlcal) flow 89-90 Transcurrent fault 214 Transform fault 213,219 Transgression 269 Transgressive systems tract 275-277 Transgressive-regressive (T-R) cycles 257 Transmutation of atoms 245 TransportatIOn of mmeral grams 98 Transportational-deposItional conditions 68 Transpresslve movement 219 Transtensile movement 2 I 9 Transverse dune 169,170, 172 Trend surface analysIs 285 Tnpple JunctlOn 217, 239 TruncatIOn 250 Tubarao, Brazil 168 Tufa 36 Tuffs 52 Turbidite 157-58, 178, 209 TurbidIty (density) current 27,88, 149, ISS-56 Turbidity flow process 155-156 Turbulent flow 89,90-91 Turnover (of Lake) 174

u Udden-Wentworth size scale 61 Unconformity 248, 250 Undathem 149, 153 Usn River 74, 114, 181

V Vadose zone 56-57, 129 Valley glacier 164-65 Varve 164, 166 Vector properties 283 Velocity pressure 95 profile 94 Vempalle Formation 40 venkatpur sandstone 173

Vertical and lateral (stratigraphic) relatIOnships 248-251 Vertlsol 132 Vmdhyan Basm 223, 225-226-28, 234,240 Vmdhyan Supergroup (system) I sediments 23-24, 31,53,145-46,177 VISCOSity 21, 88 coeffiCient 89 Volcamc breCCia 20, 52 VolcamclastlC sediments 52-53 Volcamsts 3 Vortex npples 114-15 Vuggy porOSity 58

w Wacke 21,25,27 Wackestone 39, 43,152,198 Walther's law 251-254, 259 WalVIS bay, S Africa 171 Wash load (dust load) 95-96 Washover flats 19192 Water escape structures 124 Wave npple structures 113 Wavy beddmg 108 Weathenng 12 Weight frequency 65-66, 82 West Bengal BaSIn 226, 241, 244 West Canada Basm 223, 225·26, 234,240 Wlrelme loggmg 260·262·263·266, 292-294, 297 Worm burrows 198 Wnnkle marks 11 8-19

x Xenotoplc fabnc 45 X-ray dlffracMn (XRD) 5 X-ray fluorescence (XRF) 5

y YorkshIre LImestone 131

z Zmgg's shape classificatIOn 77

t 0004786

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